diff --git a/messages/de.json b/messages/de.json
index 62e7dfdaa..b8e7cd85c 100644
--- a/messages/de.json
+++ b/messages/de.json
@@ -2301,6 +2301,11 @@
       "fastingObserved": "Nüchternglukose: {value} {unit}",
       "fastingReference": "{low} bis {high} {unit} nüchtern",
       "personalReference": "Dein Ziel: {value}",
+      "bpBasisAbove": "Ausschlaggebend: {axis}, etwa {offset} mmHg über der Obergrenze von {boundary} mmHg.",
+      "bpBasisBelowFloor": "Ausschlaggebend: {axis}, etwa {offset} mmHg unter der Grenze für niedrigen Blutdruck von {boundary} mmHg.",
+      "bpBasisInBand": "Beide Werte liegen auf oder unter ihren Referenz-Obergrenzen.",
+      "bpScale": "An der Obergrenze liegt der Wert bei 85; 100 bedeutet rund 12 mmHg darunter.",
+      "bpRepresentative": "Beruht auf einem Durchschnitt der letzten {days} Tage, der neuere Messungen stärker gewichtet, und kann deshalb von jeder einzelnen Messung abweichen.",
       "source": "Quelle: {source}, Stand {date}",
       "observedActivity": "{value} Schritte pro Tag",
       "observedSleep": "{value} Minuten pro Nacht",
diff --git a/messages/en.json b/messages/en.json
index 9c9b3db53..c47ffad22 100644
--- a/messages/en.json
+++ b/messages/en.json
@@ -2301,6 +2301,11 @@
       "fastingObserved": "Fasting glucose: {value} {unit}",
       "fastingReference": "{low} to {high} {unit} fasting",
       "personalReference": "Your target: {value}",
+      "bpBasisAbove": "{axis} sets this score: about {offset} mmHg over its ceiling of {boundary} mmHg.",
+      "bpBasisBelowFloor": "{axis} sets this score: about {offset} mmHg under the low-pressure floor of {boundary} mmHg.",
+      "bpBasisInBand": "Both values are at or below their reference ceilings.",
+      "bpScale": "At the ceiling the score is 85; 100 means roughly 12 mmHg below it.",
+      "bpRepresentative": "Based on a recency-weighted average of the last {days} days, so it may not match any single reading.",
       "source": "Source: {source}, as of {date}",
       "observedActivity": "{value} steps per day",
       "observedSleep": "{value} minutes per night",
diff --git a/messages/es.json b/messages/es.json
index b9efc2096..ef080d594 100644
--- a/messages/es.json
+++ b/messages/es.json
@@ -2301,6 +2301,11 @@
       "fastingObserved": "Glucosa en ayunas: {value} {unit}",
       "fastingReference": "{low} a {high} {unit} en ayunas",
       "personalReference": "Tu objetivo: {value}",
+      "bpBasisAbove": "Determinante: {axis}, unos {offset} mmHg por encima de su límite de {boundary} mmHg.",
+      "bpBasisBelowFloor": "Determinante: {axis}, unos {offset} mmHg por debajo del límite de presión baja de {boundary} mmHg.",
+      "bpBasisInBand": "Ambos valores están en su límite de referencia o por debajo.",
+      "bpScale": "En el límite la puntuación es 85; 100 significa unos 12 mmHg por debajo.",
+      "bpRepresentative": "Se basa en un promedio de los últimos {days} días que pondera más las mediciones recientes, por lo que puede no coincidir con ninguna medición concreta.",
       "source": "Fuente: {source}, a fecha de {date}",
       "observedActivity": "{value} pasos al día",
       "observedSleep": "{value} minutos por noche",
diff --git a/messages/fr.json b/messages/fr.json
index 69258a2dd..e229906e1 100644
--- a/messages/fr.json
+++ b/messages/fr.json
@@ -2301,6 +2301,11 @@
       "fastingObserved": "Glycémie à jeun : {value} {unit}",
       "fastingReference": "{low} à {high} {unit} à jeun",
       "personalReference": "Votre objectif : {value}",
+      "bpBasisAbove": "Déterminant : {axis}, environ {offset} mmHg au-dessus de son plafond de {boundary} mmHg.",
+      "bpBasisBelowFloor": "Déterminant : {axis}, environ {offset} mmHg sous le seuil de basse pression de {boundary} mmHg.",
+      "bpBasisInBand": "Les deux valeurs sont à leur plafond de référence ou en dessous.",
+      "bpScale": "Au plafond, le score est de 85 ; 100 correspond à environ 12 mmHg en dessous.",
+      "bpRepresentative": "Repose sur une moyenne des {days} derniers jours qui pèse davantage les mesures récentes, et peut donc ne correspondre à aucune mesure isolée.",
       "source": "Source : {source}, au {date}",
       "observedActivity": "{value} pas par jour",
       "observedSleep": "{value} minutes par nuit",
diff --git a/messages/it.json b/messages/it.json
index b14a7c282..3974df24a 100644
--- a/messages/it.json
+++ b/messages/it.json
@@ -2301,6 +2301,11 @@
       "fastingObserved": "Glicemia a digiuno: {value} {unit}",
       "fastingReference": "Da {low} a {high} {unit} a digiuno",
       "personalReference": "Il tuo obiettivo: {value}",
+      "bpBasisAbove": "Determinante: {axis}, circa {offset} mmHg sopra il suo limite di {boundary} mmHg.",
+      "bpBasisBelowFloor": "Determinante: {axis}, circa {offset} mmHg sotto la soglia di pressione bassa di {boundary} mmHg.",
+      "bpBasisInBand": "Entrambi i valori sono pari o inferiori ai loro limiti di riferimento.",
+      "bpScale": "Al limite il punteggio è 85; 100 significa circa 12 mmHg più in basso.",
+      "bpRepresentative": "Si basa su una media degli ultimi {days} giorni che pesa di più le misurazioni recenti, quindi può non corrispondere a nessuna singola misurazione.",
       "source": "Fonte: {source}, al {date}",
       "observedActivity": "{value} passi al giorno",
       "observedSleep": "{value} minuti per notte",
diff --git a/messages/pl.json b/messages/pl.json
index 51f7d586f..c49b93879 100644
--- a/messages/pl.json
+++ b/messages/pl.json
@@ -2301,6 +2301,11 @@
       "fastingObserved": "Glukoza na czczo: {value} {unit}",
       "fastingReference": "Od {low} do {high} {unit} na czczo",
       "personalReference": "Twój cel: {value}",
+      "bpBasisAbove": "Decyduje: {axis}, około {offset} mmHg powyżej swojej granicy {boundary} mmHg.",
+      "bpBasisBelowFloor": "Decyduje: {axis}, około {offset} mmHg poniżej granicy niskiego ciśnienia {boundary} mmHg.",
+      "bpBasisInBand": "Obie wartości są na poziomie swoich granic referencyjnych lub poniżej.",
+      "bpScale": "Przy granicy wynik to 85; 100 oznacza około 12 mmHg poniżej.",
+      "bpRepresentative": "Opiera się na średniej z ostatnich {days} dni, która mocniej waży nowsze pomiary, więc może nie odpowiadać żadnemu pojedynczemu pomiarowi.",
       "source": "Źródło: {source}, stan na {date}",
       "observedActivity": "{value} kroków dziennie",
       "observedSleep": "{value} minut na noc",
diff --git a/src/app/api/analytics/__tests__/route.test.ts b/src/app/api/analytics/__tests__/route.test.ts
index 8f7b11e3e..44a4903f7 100644
--- a/src/app/api/analytics/__tests__/route.test.ts
+++ b/src/app/api/analytics/__tests__/route.test.ts
@@ -450,3 +450,183 @@ describe("GET /api/analytics", () => {
   // contract (the chunked findMany must never re-appear on the default
   // slice critical path).
 });
+
+/**
+ * v1.34.5 — the blood-pressure score's basis has to ARRIVE, not merely exist.
+ *
+ * The grader returns `{ score, basis }` and the pillar builder copies the
+ * basis onto the pillar value, and both of those ends have their own unit
+ * tests. Neither of them sees the assembly in between: the route's envelope
+ * hand-off, the score reader's envelope narrowing, the pillar list, the JSON
+ * envelope. A field can be produced correctly, consumed correctly, and still
+ * never reach a client, which is precisely how v1.33.1 shipped a payload
+ * field that nothing on the wire ever carried.
+ *
+ * So these drive the REAL route. Nothing about the score is stubbed — the
+ * fixture seeds raw blood-pressure rows on the same `$queryRawUnsafe` the
+ * live BP path issues, and the route does its own pairing, its own
+ * recency-weighting, its own grading and its own serialisation. The whole
+ * payload is read back off `res.json()`.
+ */
+describe("GET /api/analytics — blood-pressure score basis on the wire", () => {
+  const DAY_MS = 24 * 60 * 60 * 1000;
+
+  /**
+   * ESH 2023 band for the fixture's date of birth (age < 65).
+   */
+  const CLINICAL = { sysHigh: 129, diaHigh: 79 } as const;
+
+  interface WirePillar {
+    id: string;
+    result: {
+      status: string;
+      value?: {
+        score: number;
+        scoreBasis?: {
+          axis: string;
+          relation: string;
+          offsetMmHg: number;
+          boundaryMmHg: number;
+        };
+        observed: { label: string };
+        reference: { label: string };
+        personalReference?: { label: string };
+      };
+    };
+  }
+
+  /**
+   * Seed one blood-pressure pair per day over the trailing `pairs` days.
+   * Every pair carries the same reading, so the recency-weighted
+   * representative is that reading exactly and the expected score can be
+   * stated as a literal instead of a tolerance.
+   */
+  function seedBpSeries(input: { sys: number; dia: number; pairs: number }) {
+    const seriesOf = (value: number) =>
+      Array.from({ length: input.pairs }, (_, i) => ({
+        measured_at: new Date(Date.now() - (i + 1) * DAY_MS),
+        value,
+      }));
+    const sysRows = seriesOf(input.sys);
+    const diaRows = seriesOf(input.dia);
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    vi.mocked(prisma.$queryRawUnsafe as any).mockImplementation(
+      async (sql: unknown) => {
+        if (typeof sql !== "string") return [];
+        if (sql.includes(`m."type" = 'BLOOD_PRESSURE_SYS'`)) return sysRows;
+        if (sql.includes(`m."type" = 'BLOOD_PRESSURE_DIA'`)) return diaRows;
+        return [];
+      },
+    );
+  }
+
+  function signInWith(thresholdsJson: unknown) {
+    vi.mocked(getSession).mockResolvedValue({
+      session: SESSION_OK.session,
+      user: { ...SESSION_USER, thresholdsJson },
+    } as never);
+  }
+
+  async function readBloodPressurePillar(): Promise<WirePillar> {
+    const res = await (
+      GET as unknown as (...args: never[]) => Promise<Response>
+    )();
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as {
+      data: { healthScore: { pillars: WirePillar[] } | null };
+    };
+    const pillar = body.data.healthScore?.pillars.find(
+      (p) => p.id === "BLOOD_PRESSURE",
+    );
+    if (!pillar) throw new Error("no BLOOD_PRESSURE pillar in the payload");
+    return pillar;
+  }
+
+  it("carries the basis onto the pillar the client actually receives", async () => {
+    // 137/91 against the 129/79 band: systolic sits 8 over its ceiling and
+    // grades 57, diastolic sits 12 over and grades 50. The pillar takes the
+    // worse of the two, so diastolic is the axis that set the number.
+    seedBpSeries({ sys: 137, dia: 91, pairs: 14 });
+    const pillar = await readBloodPressurePillar();
+
+    expect(pillar.result.status).toBe("ok");
+    expect(pillar.result.value?.observed.label).toBe("137/91 mmHg");
+    expect(pillar.result.value?.score).toBe(50);
+    expect(pillar.result.value?.scoreBasis).toEqual({
+      axis: "diastolic",
+      relation: "above_ceiling",
+      offsetMmHg: 12,
+      boundaryMmHg: CLINICAL.diaHigh,
+    });
+  });
+
+  it("says the reading is in band when it is", async () => {
+    // 126/68 clears both ceilings, so no axis is over anything and the
+    // relation has to say so rather than reporting a distance from a ceiling
+    // the reading never reached. Systolic is the nearer of the two to its
+    // own ceiling here, so this also puts the other axis on the wire.
+    seedBpSeries({ sys: 126, dia: 68, pairs: 14 });
+    const pillar = await readBloodPressurePillar();
+
+    expect(pillar.result.value?.observed.label).toBe("126/68 mmHg");
+    expect(pillar.result.value?.scoreBasis).toEqual({
+      axis: "systolic",
+      relation: "in_band",
+      offsetMmHg: 3,
+      boundaryMmHg: CLINICAL.sysHigh,
+    });
+  });
+
+  it("omits the basis entirely when the pillar has no score to explain", async () => {
+    // Eleven pairs is one short of the floor, so the pillar is insufficient
+    // and carries no value at all — the popover must fall back to the
+    // shipped lines rather than to an explanation of a number nobody has.
+    seedBpSeries({ sys: 137, dia: 91, pairs: 11 });
+    const pillar = await readBloodPressurePillar();
+
+    expect(pillar.result.status).toBe("insufficient");
+    expect(pillar.result.value).toBeUndefined();
+  });
+
+  /**
+   * The coupling the two-ended tests cannot see.
+   *
+   * A user with a personal target gets the whole BP helper run twice: once
+   * against their own band for the in-target percentages on the dashboard,
+   * once against the clinical band for the score. The score comes from the
+   * second run. So does the basis, and it has to, because the two runs
+   * disagree on every field of it — different binding axis, different
+   * ceiling, different distance. Picking the score off one run and the
+   * basis off the other reads as correct in review and produces a popover
+   * that explains a number the user is not looking at.
+   *
+   * The fixture is chosen so the two runs share NOTHING: 137/91 against the
+   * personal 135/95 band binds on systolic 2 over 135 and scores 77; against
+   * the clinical 129/79 band it binds on diastolic 12 over 79 and scores 50.
+   */
+  it("takes the basis from the same run as the score when a personal target differs", async () => {
+    signInWith({
+      BLOOD_PRESSURE_SYS: { min: 110, max: 135 },
+      BLOOD_PRESSURE_DIA: { min: 65, max: 95 },
+    });
+    seedBpSeries({ sys: 137, dia: 91, pairs: 14 });
+    const pillar = await readBloodPressurePillar();
+
+    // The personal band is genuinely in play — this account really does take
+    // the route's second-run branch, so the assertions below are about the
+    // coupling and not about a user who has no personal target at all.
+    expect(pillar.result.value?.personalReference?.label).toBe(
+      "110–135/65–95 mmHg",
+    );
+
+    expect(pillar.result.value?.score).toBe(50);
+    expect(pillar.result.value?.scoreBasis).toEqual({
+      axis: "diastolic",
+      relation: "above_ceiling",
+      offsetMmHg: 12,
+      boundaryMmHg: CLINICAL.diaHigh,
+    });
+    // Stated negatively as well: nothing from the personal run may leak in.
+    expect(pillar.result.value?.scoreBasis?.boundaryMmHg).not.toBe(135);
+  });
+});
diff --git a/src/app/api/analytics/route.ts b/src/app/api/analytics/route.ts
index aebd44534..ca9b9c5ad 100644
--- a/src/app/api/analytics/route.ts
+++ b/src/app/api/analytics/route.ts
@@ -360,6 +360,17 @@ async function buildAnalyticsResponse(user: AuthedUser, locale: Locale) {
         }),
       ],
     );
+    // The pillar is graded against the CLINICAL band even when the user
+    // keeps a personal target (that band is the number's identity; the
+    // personal one is displayed beside it and scores nothing). So when the
+    // two differ we re-run the whole helper against the clinical targets
+    // and the score comes from that second run.
+    //
+    // v1.34.5 — the "why this number" basis rides the same envelope, so it
+    // is the same run, the same representative pair and the same ceilings
+    // by construction. Assign envelopes WHOLE here; picking `graded.score`
+    // off one run and a basis off the other would let the popover explain
+    // a number nobody is looking at, and it would look correct in review.
     if (bpTargetsEqual(bpTargets, clinicalBpTargets)) {
       bpEnvelope = windows;
       bpEnvelopePriorWeek = windowsPriorWeek;
diff --git a/src/components/insights/__tests__/health-score-pillar-detail-bp-basis.test.tsx b/src/components/insights/__tests__/health-score-pillar-detail-bp-basis.test.tsx
new file mode 100644
index 000000000..6c952ca25
--- /dev/null
+++ b/src/components/insights/__tests__/health-score-pillar-detail-bp-basis.test.tsx
@@ -0,0 +1,208 @@
+/**
+ * v1.34.5 — the route from the blood-pressure score to its reason.
+ *
+ * A pillar reading 56 gave no way to find out why: the popover held the
+ * reference band, the personal target and the source, and nothing said
+ * which of the two axes had set the number, how far over its ceiling that
+ * axis sat, or that a reading exactly at the ceiling reads 85 rather than
+ * 100. These pin the three lines that answer it, in the two locales the
+ * maintainer reads, against the real bundles.
+ *
+ * The lines live behind a Radix popover, which only mounts once opened,
+ * so they are asserted through the pure formatter the card feeds it —
+ * the same strings the card shows, not a second copy of the copy.
+ */
+import { describe, expect, it } from "vitest";
+import { renderToStaticMarkup } from "react-dom/server";
+
+import { I18nProvider, useTranslations } from "@/lib/i18n/context";
+import type { Locale } from "@/lib/i18n/config";
+import type { BpScoreBasis } from "@/lib/analytics/bp-grade";
+import type { ScorePillarResult } from "@/lib/analytics/score/types";
+import {
+  pillarDetailLines,
+  type Translate,
+} from "../health-score-pillar-detail";
+
+function translator(locale: Locale = "en"): Translate {
+  let captured: Translate | null = null;
+  function Probe() {
+    captured = useTranslations().t;
+    return null;
+  }
+  renderToStaticMarkup(
+    <I18nProvider initialLocale={locale}>
+      <Probe />
+    </I18nProvider>,
+  );
+  if (!captured) throw new Error("translator probe did not run");
+  return captured;
+}
+
+function bloodPressure(scoreBasis?: BpScoreBasis): ScorePillarResult {
+  return {
+    id: "BLOOD_PRESSURE",
+    domain: "cardiometabolic",
+    result: {
+      status: "ok",
+      value: {
+        score: 56,
+        ...(scoreBasis ? { scoreBasis } : {}),
+        observed: {
+          value: 137,
+          unit: "mmHg",
+          label: "137/87 mmHg",
+          asOf: "2026-07-30T08:00:00.000Z",
+          sources: ["MANUAL"],
+        },
+        reference: {
+          kind: "clinical-threshold",
+          low: 120,
+          high: 129,
+          label: "120 to 129/70 to 79 mmHg",
+          source: "ESH 2023",
+        },
+        noiseFloor: 1,
+        deltaEligible: true,
+        deltaIdentity: "graded_bp_pair_series",
+      },
+      coverage: {
+        requiredInputs: 12,
+        presentInputs: 12,
+        historyDays: 60,
+        missing: [],
+      },
+      confidence: { score: 100, band: "high" },
+      provenance: {
+        inputs: ["BLOOD_PRESSURE"],
+        source: "live",
+        windowDays: 90,
+        computedAt: "2026-07-30T12:00:00.000Z",
+      },
+    },
+  };
+}
+
+const ABOVE_CEILING: BpScoreBasis = {
+  axis: "systolic",
+  relation: "above_ceiling",
+  offsetMmHg: 8,
+  boundaryMmHg: 129,
+};
+
+describe("blood-pressure score basis in the popover", () => {
+  it("leads with the binding axis, its distance and its ceiling", () => {
+    const lines = pillarDetailLines(bloodPressure(ABOVE_CEILING), {
+      t: translator(),
+      glucoseUnit: "mg/dL",
+    });
+    expect(lines[0]).toBe(
+      "Systolic sets this score: about 8 mmHg over its ceiling of 129 mmHg.",
+    );
+  });
+
+  it("names the diastolic axis when that is the one that bound", () => {
+    const lines = pillarDetailLines(
+      bloodPressure({
+        axis: "diastolic",
+        relation: "above_ceiling",
+        offsetMmHg: 9,
+        boundaryMmHg: 79,
+      }),
+      { t: translator(), glucoseUnit: "mg/dL" },
+    );
+    expect(lines[0]).toBe(
+      "Diastolic sets this score: about 9 mmHg over its ceiling of 79 mmHg.",
+    );
+  });
+
+  it("measures a hypotensive reading from the low-pressure floor", () => {
+    const lines = pillarDetailLines(
+      bloodPressure({
+        axis: "systolic",
+        relation: "below_floor",
+        offsetMmHg: 4,
+        boundaryMmHg: 90,
+      }),
+      { t: translator(), glucoseUnit: "mg/dL" },
+    );
+    expect(lines[0]).toBe(
+      "Systolic sets this score: about 4 mmHg under the low-pressure floor of 90 mmHg.",
+    );
+  });
+
+  it("says both axes are in band without naming a distance", () => {
+    const lines = pillarDetailLines(
+      bloodPressure({
+        axis: "systolic",
+        relation: "in_band",
+        offsetMmHg: 5,
+        boundaryMmHg: 129,
+      }),
+      { t: translator(), glucoseUnit: "mg/dL" },
+    );
+    expect(lines[0]).toBe(
+      "Both values are at or below their reference ceilings.",
+    );
+  });
+
+  it("explains the scale and the averaged reading, then the shipped lines", () => {
+    const lines = pillarDetailLines(bloodPressure(ABOVE_CEILING), {
+      t: translator(),
+      glucoseUnit: "mg/dL",
+    });
+    expect(lines[1]).toBe(
+      "At the ceiling the score is 85; 100 means roughly 12 mmHg below it.",
+    );
+    // The window comes off the pillar's own provenance, so the sentence
+    // cannot drift from the window the score was actually read over.
+    expect(lines[2]).toBe(
+      "Based on a recency-weighted average of the last 90 days, so it may not match any single reading.",
+    );
+    expect(lines).toHaveLength(5);
+    expect(lines[3]).toContain("ESH 2023");
+    expect(lines[4]).toContain("Source: MANUAL");
+  });
+
+  it("reads in German too", () => {
+    const lines = pillarDetailLines(bloodPressure(ABOVE_CEILING), {
+      t: translator("de"),
+      glucoseUnit: "mg/dL",
+    });
+    expect(lines[0]).toBe(
+      "Ausschlaggebend: Systolisch, etwa 8 mmHg über der Obergrenze von 129 mmHg.",
+    );
+    expect(lines[1]).toBe(
+      "An der Obergrenze liegt der Wert bei 85; 100 bedeutet rund 12 mmHg darunter.",
+    );
+    expect(lines[2]).toBe(
+      "Beruht auf einem Durchschnitt der letzten 90 Tage, der neuere Messungen stärker gewichtet, und kann deshalb von jeder einzelnen Messung abweichen.",
+    );
+  });
+
+  it("says nothing at all when the server published no basis", () => {
+    // The field is optional and blood-pressure-only: an older payload, or
+    // a pillar that has nothing of the kind to say, must fall back to the
+    // shipped lines rather than to an empty or invented sentence.
+    const lines = pillarDetailLines(bloodPressure(), {
+      t: translator(),
+      glucoseUnit: "mg/dL",
+    });
+    expect(lines).toHaveLength(2);
+    expect(lines[0]).toContain("ESH 2023");
+    expect(lines.join("\n")).not.toContain("mmHg over its ceiling");
+  });
+
+  it("leaves other pillars untouched", () => {
+    const glycaemia: ScorePillarResult = {
+      ...bloodPressure(ABOVE_CEILING),
+      id: "GLYCAEMIA",
+    };
+    const lines = pillarDetailLines(glycaemia, {
+      t: translator(),
+      glucoseUnit: "mg/dL",
+    });
+    expect(lines.join("\n")).not.toContain("sets this score");
+    expect(lines.join("\n")).not.toContain("At the ceiling");
+  });
+});
diff --git a/src/components/insights/health-score-pillar-detail.ts b/src/components/insights/health-score-pillar-detail.ts
index 27cb4c5fa..f947bd106 100644
--- a/src/components/insights/health-score-pillar-detail.ts
+++ b/src/components/insights/health-score-pillar-detail.ts
@@ -152,10 +152,59 @@ function pillarReferenceText(
 }
 
 /**
- * The lines behind the cell's info popover: the scored reference, the
- * user-authored target when one exists, and the source with its as-of date.
- * Ordered as they render; an absent personal target contributes no line
- * rather than an empty one.
+ * Blood pressure only: how the number came about.
+ *
+ * The BP pillar scores the WORSE of the two axes against a reference
+ * ceiling, so the score on its own cannot say which axis it came from,
+ * how far over the ceiling that axis sat, or why a reading sitting
+ * exactly at the ceiling reads 85 rather than 100. All three were
+ * invisible, which is what made a defensible number feel arbitrary.
+ *
+ * The basis is resolved on the server and travels with the score, so
+ * nothing here recomputes anything: this reads `scoreBasis` and writes
+ * sentences. A pillar without a basis contributes no lines at all.
+ */
+function bpBasisLines(
+  pillar: ScorePillarResult,
+  { t }: PillarDetailContext,
+): string[] {
+  if (pillar.id !== "BLOOD_PRESSURE" || pillar.result.status !== "ok") {
+    return [];
+  }
+  const basis = pillar.result.value.scoreBasis;
+  if (!basis) return [];
+  // `charts.systolic` / `charts.diastolic` ship in all six locales and are
+  // already the words the BP chart uses for these axes.
+  const axis =
+    basis.axis === "systolic" ? t("charts.systolic") : t("charts.diastolic");
+  const params = {
+    axis,
+    offset: basis.offsetMmHg,
+    boundary: basis.boundaryMmHg,
+  };
+  const basisLine =
+    basis.relation === "above_ceiling"
+      ? t("insights.healthScore.bpBasisAbove", params)
+      : basis.relation === "below_floor"
+        ? t("insights.healthScore.bpBasisBelowFloor", params)
+        : t("insights.healthScore.bpBasisInBand");
+  return [
+    basisLine,
+    t("insights.healthScore.bpScale"),
+    // The observed pair is a recency-weighted mean over the pillar's own
+    // window, not a reading anyone ever took. Saying so is what keeps the
+    // observed line from reading as "I never had that reading".
+    t("insights.healthScore.bpRepresentative", {
+      days: pillar.result.provenance.windowDays,
+    }),
+  ];
+}
+
+/**
+ * The lines behind the cell's info popover: for blood pressure the basis
+ * of the score, then the scored reference, the user-authored target when
+ * one exists, and the source with its as-of date. Ordered as they render;
+ * an absent personal target contributes no line rather than an empty one.
  */
 export function pillarDetailLines(
   pillar: ScorePillarResult,
@@ -165,6 +214,7 @@ export function pillarDetailLines(
   const { t } = ctx;
   const value = pillar.result.value;
   const lines = [
+    ...bpBasisLines(pillar, ctx),
     t("insights.healthScore.reference", {
       value: pillarReferenceText(pillar, ctx),
     }),
diff --git a/src/lib/analytics/__tests__/bp-grade.test.ts b/src/lib/analytics/__tests__/bp-grade.test.ts
index 209e5715b..b0c6e8b6a 100644
--- a/src/lib/analytics/__tests__/bp-grade.test.ts
+++ b/src/lib/analytics/__tests__/bp-grade.test.ts
@@ -7,6 +7,8 @@
  * (low-to-mid 50s), not in the catastrophic band.
  */
 import { describe, expect, it } from "vitest";
+import { readFileSync } from "node:fs";
+import { join } from "node:path";
 import {
   gradeBpScore,
   gradeBpScoreFromSeries,
@@ -22,49 +24,58 @@ const UNDER65: BpTargets = {
   diaHigh: 79,
 };
 
+/**
+ * The score alone, for the arithmetic pins below. `gradeBpScore` returns
+ * the score together with the basis that produced it; the basis has its
+ * own describe block further down.
+ */
+function bpScore(input: { sys: number; dia: number; target: BpTargets }) {
+  return gradeBpScore(input).score;
+}
+
 describe("gradeBpScore", () => {
   it("grades the maintainer's 134/87 into the borderline band [45,60]", () => {
-    const score = gradeBpScore({ sys: 134, dia: 87, target: UNDER65 });
+    const score = bpScore({ sys: 134, dia: 87, target: UNDER65 });
     expect(score).toBeGreaterThanOrEqual(45);
     expect(score).toBeLessThanOrEqual(60);
   });
 
   it("grades a well-controlled 120/78 at or above 85", () => {
     expect(
-      gradeBpScore({ sys: 120, dia: 78, target: UNDER65 }),
+      bpScore({ sys: 120, dia: 78, target: UNDER65 }),
     ).toBeGreaterThanOrEqual(85);
   });
 
   it("grades a textbook-normal 118/76 very high (≈100)", () => {
     expect(
-      gradeBpScore({ sys: 118, dia: 76, target: UNDER65 }),
+      bpScore({ sys: 118, dia: 76, target: UNDER65 }),
     ).toBeGreaterThanOrEqual(88);
   });
 
   it("grades an uncontrolled 160/100 at or below 30", () => {
     expect(
-      gradeBpScore({ sys: 160, dia: 100, target: UNDER65 }),
+      bpScore({ sys: 160, dia: 100, target: UNDER65 }),
     ).toBeLessThanOrEqual(30);
   });
 
   it("takes the WORSE axis — a single high diastolic drags the score down", () => {
     // sys perfect (118), dia far over (95): the worst axis should win.
-    const sysOk = gradeBpScore({ sys: 118, dia: 78, target: UNDER65 });
-    const diaHigh = gradeBpScore({ sys: 118, dia: 95, target: UNDER65 });
+    const sysOk = bpScore({ sys: 118, dia: 78, target: UNDER65 });
+    const diaHigh = bpScore({ sys: 118, dia: 95, target: UNDER65 });
     expect(diaHigh).toBeLessThan(sysOk);
   });
 
   it("is monotonic — higher BP never scores better", () => {
-    const a = gradeBpScore({ sys: 130, dia: 82, target: UNDER65 });
-    const b = gradeBpScore({ sys: 140, dia: 88, target: UNDER65 });
-    const c = gradeBpScore({ sys: 150, dia: 95, target: UNDER65 });
+    const a = bpScore({ sys: 130, dia: 82, target: UNDER65 });
+    const b = bpScore({ sys: 140, dia: 88, target: UNDER65 });
+    const c = bpScore({ sys: 150, dia: 95, target: UNDER65 });
     expect(a).toBeGreaterThan(b);
     expect(b).toBeGreaterThan(c);
   });
 
   it("has no cliff — a one-mmHg change moves the score by a small amount", () => {
-    const at = gradeBpScore({ sys: 129, dia: 79, target: UNDER65 });
-    const justOver = gradeBpScore({ sys: 130, dia: 79, target: UNDER65 });
+    const at = bpScore({ sys: 129, dia: 79, target: UNDER65 });
+    const justOver = bpScore({ sys: 130, dia: 79, target: UNDER65 });
     expect(at - justOver).toBeLessThanOrEqual(5);
     expect(at).toBeGreaterThanOrEqual(justOver);
   });
@@ -73,11 +84,11 @@ describe("gradeBpScore", () => {
     // 85/45 is 5 mmHg below both floors (sys 90 / dia 50). The continuous
     // hypo curve gives a gentle penalty near the floor (5 below ≈ 85),
     // clearly below the optimal plateau of 100.
-    const low = gradeBpScore({ sys: 85, dia: 45, target: UNDER65 });
+    const low = bpScore({ sys: 85, dia: 45, target: UNDER65 });
     expect(low).toBeLessThan(100);
     expect(low).toBeGreaterThanOrEqual(80);
     // A markedly low reading is penalised much harder.
-    const veryLow = gradeBpScore({ sys: 70, dia: 40, target: UNDER65 });
+    const veryLow = bpScore({ sys: 70, dia: 40, target: UNDER65 });
     expect(veryLow).toBeLessThan(low);
     expect(veryLow).toBeLessThanOrEqual(45);
   });
@@ -90,7 +101,7 @@ describe("gradeBpScore", () => {
     // dia 66 sits on the optimal plateau (offset −13 → 100) so the
     // systolic axis is the worst-of(sys,dia) winner across the walk.
     const sysWalk = [92, 91, 90, 89, 88].map((sys) =>
-      gradeBpScore({ sys, dia: 66, target: UNDER65 }),
+      bpScore({ sys, dia: 66, target: UNDER65 }),
     );
     // AT and ABOVE the floor sit on the plateau (100).
     expect(sysWalk[2]).toBe(100); // sys 90
@@ -109,7 +120,7 @@ describe("gradeBpScore", () => {
     // sys 117 sits on the optimal plateau (offset −12 → 100) so the
     // diastolic axis is the worst-of(sys,dia) winner across the walk.
     const diaWalk = [52, 51, 50, 49, 48].map((dia) =>
-      gradeBpScore({ sys: 117, dia, target: UNDER65 }),
+      bpScore({ sys: 117, dia, target: UNDER65 }),
     );
     expect(diaWalk[2]).toBe(100); // dia 50
     expect(diaWalk[1]).toBe(100); // dia 51
@@ -124,19 +135,19 @@ describe("gradeBpScore", () => {
   it("descends smoothly further below the floor (mirrors over-target steepness)", () => {
     // floor → 100, floor−10 → ~70, floor−20 → ~45, floor−30 → ~20.
     // dia 66 stays on the plateau so the systolic axis is the winner.
-    expect(gradeBpScore({ sys: 90, dia: 66, target: UNDER65 })).toBe(100);
-    expect(gradeBpScore({ sys: 80, dia: 66, target: UNDER65 })).toBe(70);
-    expect(gradeBpScore({ sys: 70, dia: 66, target: UNDER65 })).toBe(45);
-    expect(gradeBpScore({ sys: 60, dia: 66, target: UNDER65 })).toBe(20);
+    expect(bpScore({ sys: 90, dia: 66, target: UNDER65 })).toBe(100);
+    expect(bpScore({ sys: 80, dia: 66, target: UNDER65 })).toBe(70);
+    expect(bpScore({ sys: 70, dia: 66, target: UNDER65 })).toBe(45);
+    expect(bpScore({ sys: 60, dia: 66, target: UNDER65 })).toBe(20);
     // Never negative on an extreme low.
     expect(
-      gradeBpScore({ sys: 30, dia: 20, target: UNDER65 }),
+      bpScore({ sys: 30, dia: 20, target: UNDER65 }),
     ).toBeGreaterThanOrEqual(0);
   });
 
   it("preserves the audit fairness anchors (134/87 → 57, 165/105 → 22)", () => {
-    expect(gradeBpScore({ sys: 134, dia: 87, target: UNDER65 })).toBe(57);
-    expect(gradeBpScore({ sys: 165, dia: 105, target: UNDER65 })).toBe(22);
+    expect(bpScore({ sys: 134, dia: 87, target: UNDER65 })).toBe(57);
+    expect(bpScore({ sys: 165, dia: 105, target: UNDER65 })).toBe(22);
   });
 });
 
@@ -169,7 +180,7 @@ describe("gradeBpScoreFromSeries", () => {
     // recency-weighted representative should read close to the recent
     // well-controlled cluster.
     expect(recencyWeighted).not.toBeNull();
-    expect(recencyWeighted as number).toBeGreaterThanOrEqual(80);
+    expect(recencyWeighted!.score).toBeGreaterThanOrEqual(80);
   });
 
   it("matches the single-reading grade when every pair is today", () => {
@@ -177,9 +188,9 @@ describe("gradeBpScoreFromSeries", () => {
       { at: NOW, sys: 134, dia: 87 },
       { at: NOW, sys: 134, dia: 87 },
     ];
-    expect(gradeBpScoreFromSeries({ pairs, target: UNDER65, now: NOW })).toBe(
-      gradeBpScore({ sys: 134, dia: 87, target: UNDER65 }),
-    );
+    expect(
+      gradeBpScoreFromSeries({ pairs, target: UNDER65, now: NOW }),
+    ).toEqual(gradeBpScore({ sys: 134, dia: 87, target: UNDER65 }));
   });
 
   it("rollup (per-day-mean + count) and live (per-event) agree on the same data", () => {
@@ -218,8 +229,300 @@ describe("gradeBpScoreFromSeries", () => {
     });
     expect(live).not.toBeNull();
     expect(rollup).not.toBeNull();
-    expect(Math.abs((live as number) - (rollup as number))).toBeLessThanOrEqual(
-      1,
-    );
+    expect(Math.abs(live!.score - rollup!.score)).toBeLessThanOrEqual(1);
   });
 });
+
+/**
+ * v1.34.5 — the basis published with the score. The pillar scores the
+ * WORSE of the two axes; these pin which axis that was, what it was
+ * measured against, and that the three relations the popover can render
+ * are the only three that exist.
+ */
+describe("gradeBpScore — the basis", () => {
+  // Hypotension floors, mirrored from `bp-in-target` so the sweep below
+  // can reconstruct the boundary the grader must have used.
+  const SYS_FLOOR = 90;
+  const DIA_FLOOR = 50;
+
+  it("names the systolic axis when it is the worse one", () => {
+    // 137/84: sys 8 over its ceiling, dia 5 over its own — systolic worse.
+    const { score, basis } = gradeBpScore({
+      sys: 137,
+      dia: 84,
+      target: UNDER65,
+    });
+    expect(basis).toEqual({
+      axis: "systolic",
+      relation: "above_ceiling",
+      offsetMmHg: 8,
+      boundaryMmHg: 129,
+    });
+    expect(score).toBe(57);
+  });
+
+  it("names the diastolic axis when it is the worse one", () => {
+    // 132/88: sys 3 over, dia 9 over — diastolic worse, and the basis
+    // must say so rather than defaulting to the axis people read first.
+    const { score, basis } = gradeBpScore({
+      sys: 132,
+      dia: 88,
+      target: UNDER65,
+    });
+    expect(basis).toEqual({
+      axis: "diastolic",
+      relation: "above_ceiling",
+      offsetMmHg: 9,
+      boundaryMmHg: 79,
+    });
+    expect(score).toBe(55);
+  });
+
+  it("breaks a tie toward systolic", () => {
+    // Both axes sit exactly 5 mmHg over their own ceiling, so both grade
+    // 65 and the score cannot say which one it came from. The documented
+    // rule picks systolic.
+    const { basis } = gradeBpScore({ sys: 134, dia: 84, target: UNDER65 });
+    expect(basis.axis).toBe("systolic");
+    expect(basis.boundaryMmHg).toBe(129);
+    // …and at the ceiling itself, where both axes grade 85.
+    expect(gradeBpScore({ sys: 129, dia: 79, target: UNDER65 }).basis).toEqual({
+      axis: "systolic",
+      relation: "in_band",
+      offsetMmHg: 0,
+      boundaryMmHg: 129,
+    });
+  });
+
+  it("reports a reading inside the band as in_band, measured from the ceiling", () => {
+    const { score, basis } = gradeBpScore({
+      sys: 124,
+      dia: 74,
+      target: UNDER65,
+    });
+    expect(basis.relation).toBe("in_band");
+    expect(basis.boundaryMmHg).toBe(129);
+    expect(basis.offsetMmHg).toBe(5); // five below the ceiling
+    expect(score).toBeGreaterThanOrEqual(85);
+  });
+
+  it("reports a hypotensive reading as below_floor, measured from the floor", () => {
+    // 86/70: systolic 4 under the 90 floor; diastolic comfortably in band.
+    const { basis } = gradeBpScore({ sys: 86, dia: 70, target: UNDER65 });
+    expect(basis).toEqual({
+      axis: "systolic",
+      relation: "below_floor",
+      offsetMmHg: 4,
+      boundaryMmHg: SYS_FLOOR,
+    });
+  });
+
+  it("keeps below_floor even when the score is still high — the curve is gentle near the floor", () => {
+    // Worth pinning because it is easy to assume "below the floor" implies
+    // a poor score: two mmHg under reads 94, comfortably green. The
+    // relation is decided by where the value sits, never by the score.
+    const { score, basis } = gradeBpScore({
+      sys: 88,
+      dia: 70,
+      target: UNDER65,
+    });
+    expect(basis.relation).toBe("below_floor");
+    expect(score).toBeGreaterThanOrEqual(85);
+  });
+
+  it("keeps the relation with the score when the rounded offset reads zero", () => {
+    // 129.4 is above the ceiling and scores below 85, but rounds to a
+    // 0 mmHg offset. The relation follows the unrounded value, so the
+    // sentence can never claim "in band" about a sub-85 number.
+    const { score, basis } = gradeBpScore({
+      sys: 129.4,
+      dia: 70,
+      target: UNDER65,
+    });
+    expect(basis.relation).toBe("above_ceiling");
+    expect(basis.offsetMmHg).toBe(0);
+    expect(score).toBeLessThan(85);
+  });
+
+  it("reproduces the worked 137.5/87 example — systolic, ~9 over, 56", () => {
+    const { score, basis } = gradeBpScore({
+      sys: 137.5,
+      dia: 87,
+      target: UNDER65,
+    });
+    expect(score).toBe(56);
+    expect(basis).toEqual({
+      axis: "systolic",
+      relation: "above_ceiling",
+      offsetMmHg: 9,
+      boundaryMmHg: 129,
+    });
+  });
+
+  it("uses the targets it was handed, not a fixed band", () => {
+    const over65: BpTargets = {
+      sysLow: 130,
+      sysHigh: 139,
+      diaLow: 70,
+      diaHigh: 79,
+    };
+    const { basis } = gradeBpScore({ sys: 137, dia: 70, target: over65 });
+    expect(basis).toEqual({
+      axis: "systolic",
+      relation: "in_band",
+      offsetMmHg: 2,
+      boundaryMmHg: 139,
+    });
+  });
+
+  it("has exactly three relations, and each one reconstructs its own reading", () => {
+    // The design asserts the three relations are exhaustive; this pins it
+    // against the anchors instead of trusting the prose. Every integer
+    // reading across the whole plausible range must land in exactly one
+    // relation, and axis + relation + boundary + offset together must
+    // reconstruct the value that produced the score.
+    const seen = new Set<string>();
+    for (let sys = 60; sys <= 220; sys += 1) {
+      for (let dia = 30; dia <= 140; dia += 1) {
+        const { score, basis } = gradeBpScore({ sys, dia, target: UNDER65 });
+        seen.add(basis.relation);
+        const value = basis.axis === "systolic" ? sys : dia;
+        const floor = basis.axis === "systolic" ? SYS_FLOOR : DIA_FLOOR;
+        const ceiling =
+          basis.axis === "systolic" ? UNDER65.sysHigh : UNDER65.diaHigh;
+
+        if (basis.relation === "below_floor") {
+          expect(value).toBeLessThan(floor);
+          expect(basis.boundaryMmHg).toBe(floor);
+          expect(floor - value).toBe(basis.offsetMmHg);
+        } else {
+          expect(value).toBeGreaterThanOrEqual(floor);
+          expect(basis.boundaryMmHg).toBe(ceiling);
+          expect(Math.abs(value - ceiling)).toBe(basis.offsetMmHg);
+          if (basis.relation === "in_band") {
+            expect(value).toBeLessThanOrEqual(ceiling);
+            // The scale line only makes sense if in_band never reads red.
+            expect(score).toBeGreaterThanOrEqual(85);
+          } else {
+            expect(basis.relation).toBe("above_ceiling");
+            expect(value).toBeGreaterThan(ceiling);
+            expect(score).toBeLessThan(85);
+          }
+        }
+
+        // The named axis alone carries the score: replace the other axis
+        // with an optimal value and the number must not move.
+        const optimalOther = gradeBpScore(
+          basis.axis === "systolic"
+            ? { sys, dia: UNDER65.diaHigh - 12, target: UNDER65 }
+            : { sys: UNDER65.sysHigh - 12, dia, target: UNDER65 },
+        );
+        expect(optimalOther.score).toBe(score);
+      }
+    }
+    expect([...seen].sort()).toEqual([
+      "above_ceiling",
+      "below_floor",
+      "in_band",
+    ]);
+  });
+});
+
+describe("gradeBpScoreFromSeries — the basis travels with the score", () => {
+  const NOW = new Date("2026-06-07T12:00:00.000Z");
+
+  it("returns the basis of the representative it graded, not of any single pair", () => {
+    // Two readings, one at each end. The representative is their mean
+    // (both same-day, so equal weight): 130/85. The basis must describe
+    // that mean, which matches neither reading.
+    const graded = gradeBpScoreFromSeries({
+      pairs: [
+        { at: NOW, sys: 120, dia: 80 },
+        { at: NOW, sys: 140, dia: 90 },
+      ],
+      target: UNDER65,
+      now: NOW,
+    });
+    expect(graded).not.toBeNull();
+    expect(graded!.basis).toEqual({
+      axis: "diastolic",
+      relation: "above_ceiling",
+      offsetMmHg: 6,
+      boundaryMmHg: 79,
+    });
+    expect(graded!.score).toBe(
+      gradeBpScore({ sys: 130, dia: 85, target: UNDER65 }).score,
+    );
+  });
+
+  it("grades the basis against the targets it was given", () => {
+    // The analytics route re-runs this helper with the CLINICAL targets
+    // when the user keeps a personal one. Same series, two bands: both
+    // the score and the basis must move together.
+    const pairs: BpPairPoint[] = [{ at: NOW, sys: 132, dia: 88 }];
+    const clinical = gradeBpScoreFromSeries({
+      pairs,
+      target: UNDER65,
+      now: NOW,
+    })!;
+    const personal = gradeBpScoreFromSeries({
+      pairs,
+      target: { sysLow: 120, sysHigh: 135, diaLow: 70, diaHigh: 90 },
+      now: NOW,
+    })!;
+    expect(clinical.basis.boundaryMmHg).toBe(79);
+    expect(clinical.basis.relation).toBe("above_ceiling");
+    expect(personal.basis.relation).toBe("in_band");
+    expect(personal.score).toBeGreaterThan(clinical.score);
+  });
+});
+
+/**
+ * The popover's scale line quotes two numbers out of this curve ("at the
+ * ceiling the score is 85; 100 means roughly 12 mmHg below it") because
+ * the sentence has to read naturally in six languages. That makes the
+ * copy and the anchors two places that must agree, so this pins them to
+ * each other: re-anchoring the curve turns this red and the six strings
+ * have to follow.
+ */
+describe("the scale the popover quotes", () => {
+  const AT_CEILING_SCORE = 85;
+  const OPTIMAL_OFFSET_MMHG = 12;
+
+  it("still scores 85 at the ceiling and 100 at twelve below it", () => {
+    expect(
+      bpScore({ sys: UNDER65.sysHigh, dia: UNDER65.diaHigh, target: UNDER65 }),
+    ).toBe(AT_CEILING_SCORE);
+    expect(
+      bpScore({
+        sys: UNDER65.sysHigh - OPTIMAL_OFFSET_MMHG,
+        dia: UNDER65.diaHigh - OPTIMAL_OFFSET_MMHG,
+        target: UNDER65,
+      }),
+    ).toBe(100);
+    // Twelve is the SMALLEST distance that reaches 100 — one less does not.
+    expect(
+      bpScore({
+        sys: UNDER65.sysHigh - (OPTIMAL_OFFSET_MMHG - 1),
+        dia: UNDER65.diaHigh - (OPTIMAL_OFFSET_MMHG - 1),
+        target: UNDER65,
+      }),
+    ).toBeLessThan(100);
+  });
+
+  it.each(["en", "de", "fr", "es", "it", "pl"])(
+    "quotes those same two numbers in %s",
+    (locale) => {
+      const bundle = JSON.parse(
+        readFileSync(
+          join(__dirname, "../../../../messages", `${locale}.json`),
+          "utf8",
+        ),
+      ) as { insights: { healthScore: Record<string, string> } };
+      const line = bundle.insights.healthScore.bpScale;
+      expect(line).toContain(String(AT_CEILING_SCORE));
+      expect(line).toContain(String(OPTIMAL_OFFSET_MMHG));
+      expect(line).toContain("100");
+    },
+  );
+});
diff --git a/src/lib/analytics/bp-grade.ts b/src/lib/analytics/bp-grade.ts
index a1842eba2..bc809c859 100644
--- a/src/lib/analytics/bp-grade.ts
+++ b/src/lib/analytics/bp-grade.ts
@@ -84,6 +84,49 @@ const HYPO_ANCHORS: ReadonlyArray<readonly [below: number, score: number]> = [
   [40, 5],
 ] as const;
 
+/**
+ * Which axis set the score, measured against which boundary, and by how
+ * far. Published with the score (never beside it) so the surface can
+ * answer "why this number" without recomputing anything client-side.
+ *
+ * `relation` is a trichotomy on the binding axis' own value:
+ *
+ *   value <  floor                → `below_floor`   (boundary = floor)
+ *   value <= ceiling              → `in_band`       (boundary = ceiling)
+ *   otherwise                     → `above_ceiling` (boundary = ceiling)
+ *
+ * so the three relations are exhaustive and mutually exclusive by
+ * construction, whatever the anchors say. `offsetMmHg` is the distance
+ * from that boundary as a NON-NEGATIVE integer (rounded); the relation
+ * carries the direction, so the number never needs a sign. The relation
+ * is decided on the unrounded value, so it always agrees with the score
+ * even when the rounded offset reads 0.
+ */
+export type BpScoreAxis = "systolic" | "diastolic";
+export type BpScoreRelation = "above_ceiling" | "below_floor" | "in_band";
+
+export interface BpScoreBasis {
+  /** The axis whose sub-score the pillar took — the worse of the two. */
+  axis: BpScoreAxis;
+  relation: BpScoreRelation;
+  /** Rounded, non-negative distance from `boundaryMmHg`. */
+  offsetMmHg: number;
+  /** The ceiling the axis was graded against, or its hypotension floor. */
+  boundaryMmHg: number;
+}
+
+/**
+ * A graded BP reading: the score AND the basis that produced it, as one
+ * value. They are returned together, and carried together through every
+ * envelope down to the pillar, precisely so no caller can ever pair a
+ * score from one run with a basis from another (different representative
+ * pair, or — the live trap — different targets).
+ */
+export interface BpGrade {
+  score: number;
+  basis: BpScoreBasis;
+}
+
 /**
  * Generic piecewise-linear interpolation over a monotonic-x anchor table.
  * Clamps to the first / last anchor outside the table range.
@@ -127,38 +170,63 @@ function interpolateAxisScore(offset: number): number {
  *   the over-target branch holds at the floor, then descends smoothly —
  *   no boundary cliff.
  */
-function gradeAxis(value: number, ceiling: number, floor: number): number {
+function gradeAxis(
+  value: number,
+  ceiling: number,
+  floor: number,
+): { score: number; relation: BpScoreRelation; boundary: number } {
   if (value < floor) {
-    return interpolateAnchors(HYPO_ANCHORS, floor - value);
+    return {
+      score: interpolateAnchors(HYPO_ANCHORS, floor - value),
+      relation: "below_floor",
+      boundary: floor,
+    };
   }
-  return interpolateAxisScore(value - ceiling);
+  const offset = value - ceiling;
+  return {
+    score: interpolateAxisScore(offset),
+    relation: offset > 0 ? "above_ceiling" : "in_band",
+    boundary: ceiling,
+  };
 }
 
 /**
- * Map a representative blood-pressure reading to a smooth 0-100 score.
+ * Map a representative blood-pressure reading to a smooth 0-100 score,
+ * together with the basis that produced it.
  *
  * Takes the WORSE of the systolic and diastolic axis sub-scores so a
  * high diastolic counts against the score (it should) without
  * binary-zeroing the result the way the all-time in-target rate did.
  *
- * @returns integer 0-100.
+ * **Tie rule**: when both axes grade identically, the basis names the
+ * SYSTOLIC axis. The two are interchangeable for the number, so the
+ * choice is arbitrary in arithmetic but must not be arbitrary in the
+ * text; systolic is the axis clinicians lead with and the one the app
+ * shows first everywhere else.
+ *
+ * @returns integer score 0-100 plus the binding axis, its relation to
+ *          its boundary, and the distance to it.
  */
 export function gradeBpScore(input: {
   sys: number;
   dia: number;
   target: BpTargets;
-}): number {
-  const sysScore = gradeAxis(
-    input.sys,
-    input.target.sysHigh,
-    SYS_HYPOTENSION_FLOOR,
-  );
-  const diaScore = gradeAxis(
-    input.dia,
-    input.target.diaHigh,
-    DIA_HYPOTENSION_FLOOR,
-  );
-  return Math.round(Math.max(0, Math.min(100, Math.min(sysScore, diaScore))));
+}): BpGrade {
+  const sys = gradeAxis(input.sys, input.target.sysHigh, SYS_HYPOTENSION_FLOOR);
+  const dia = gradeAxis(input.dia, input.target.diaHigh, DIA_HYPOTENSION_FLOOR);
+  // `<=` puts a tie on systolic — see the tie rule above.
+  const bindingIsSystolic = sys.score <= dia.score;
+  const binding = bindingIsSystolic ? sys : dia;
+  const bindingValue = bindingIsSystolic ? input.sys : input.dia;
+  return {
+    score: Math.round(Math.max(0, Math.min(100, binding.score))),
+    basis: {
+      axis: bindingIsSystolic ? "systolic" : "diastolic",
+      relation: binding.relation,
+      offsetMmHg: Math.round(Math.abs(bindingValue - binding.boundary)),
+      boundaryMmHg: binding.boundary,
+    },
+  };
 }
 
 /**
@@ -228,11 +296,21 @@ export function representativeBpFromSeries(input: {
   };
 }
 
+/**
+ * Grade the recency-weighted representative of a pair series.
+ *
+ * Returns the score and its basis as ONE value. Callers must carry that
+ * value on, never split it: a score stored beside a basis that came from
+ * a second evaluation (or from a second set of targets — the analytics
+ * route deliberately re-runs this with the CLINICAL targets when the user
+ * has a personal one) would let the surface explain a number that is not
+ * the number on screen.
+ */
 export function gradeBpScoreFromSeries(input: {
   pairs: ReadonlyArray<BpPairPoint>;
   target: BpTargets;
   now: Date;
-}): number | null {
+}): BpGrade | null {
   const representative = representativeBpFromSeries(input);
   return representative
     ? gradeBpScore({ ...representative, target: input.target })
diff --git a/src/lib/analytics/bp-in-target-fast-path.ts b/src/lib/analytics/bp-in-target-fast-path.ts
index d754c3626..431f7def9 100644
--- a/src/lib/analytics/bp-in-target-fast-path.ts
+++ b/src/lib/analytics/bp-in-target-fast-path.ts
@@ -69,7 +69,11 @@ import {
   isBpReadingInTarget,
   type BpReading,
 } from "./bp-in-target";
-import { gradeBpScoreFromSeries, representativeBpFromSeries } from "./bp-grade";
+import {
+  gradeBpScoreFromSeries,
+  representativeBpFromSeries,
+  type BpGrade,
+} from "./bp-grade";
 import type { BpTargets } from "./bp-targets";
 
 /**
@@ -109,9 +113,16 @@ export interface BpInTargetEnvelope {
    * recency-weighted representative reading over the trailing window.
    * Feeds the Health-Score BP pillar value, replacing the binary
    * in-target rate AS THE SCORE. `null` when no pairs formed.
+   *
+   * v1.34.5 — the score and the basis that explains it travel as ONE
+   * object, from the one `gradeBpScoreFromSeries` call that produced
+   * both. The route re-runs this whole helper against the clinical
+   * targets when the user has a personal target; carrying the pair
+   * together is what makes it impossible for the explanation to belong
+   * to a different run than the number.
    */
-  gradedScore: number | null;
-  /** Same recency-weighted pair that `gradedScore` grades. */
+  graded: BpGrade | null;
+  /** Same recency-weighted pair that `graded` grades. */
   representative: { sys: number; dia: number } | null;
   /** Freshest paired reading in the trailing 90-day score window. */
   last90LatestAt: Date | null;
@@ -377,7 +388,7 @@ async function computeFromRollups(
       dia: pair.dia,
       count: pair.perDayPairCount,
     }));
-  const gradedScore = gradeBpScoreFromSeries({
+  const graded = gradeBpScoreFromSeries({
     pairs: scorePairs,
     target: targets,
     now,
@@ -398,7 +409,7 @@ async function computeFromRollups(
     priorYear,
     path: "rollup",
     rowCount,
-    gradedScore,
+    graded,
     representative,
   };
 }
@@ -545,7 +556,7 @@ async function computeFromLive(
   const scorePairs = collectBpPairs(sysData, diaData, userTz).filter(
     (pair) => pair.at >= new Date(now.getTime() - 90 * DAY_MS) && pair.at < now,
   );
-  const gradedScore = gradeBpScoreFromSeries({
+  const graded = gradeBpScoreFromSeries({
     pairs: scorePairs,
     target: targets,
     now,
@@ -568,7 +579,7 @@ async function computeFromLive(
     priorYear: windows.priorYear,
     path: "live",
     rowCount,
-    gradedScore,
+    graded,
     representative,
   };
 }
diff --git a/src/lib/analytics/score/__tests__/pillars.test.ts b/src/lib/analytics/score/__tests__/pillars.test.ts
index 2681dd4a9..15fd2eb38 100644
--- a/src/lib/analytics/score/__tests__/pillars.test.ts
+++ b/src/lib/analytics/score/__tests__/pillars.test.ts
@@ -1,5 +1,6 @@
 import { describe, expect, it } from "vitest";
 
+import { gradeBpScore } from "@/lib/analytics/bp-grade";
 import { computeActivityPillar } from "../activity";
 import { computeAdiposityPillar } from "../adiposity";
 import { computeBloodPressurePillar } from "../blood-pressure";
@@ -30,7 +31,11 @@ describe("reference-score pillars", () => {
     const below = computeBloodPressurePillar({
       ...live,
       pairCount: 11,
-      gradedScore: 85,
+      graded: gradeBpScore({
+        sys: 129,
+        dia: 79,
+        target: { sysLow: 120, sysHigh: 129, diaLow: 70, diaHigh: 79 },
+      }),
       representative: { sys: 129, dia: 79 },
       oldestAt: new Date("2026-07-01T08:00:00Z"),
       latestAt: new Date("2026-07-27T08:00:00Z"),
@@ -44,7 +49,11 @@ describe("reference-score pillars", () => {
     const eligible = computeBloodPressurePillar({
       ...live,
       pairCount: 12,
-      gradedScore: 85,
+      graded: gradeBpScore({
+        sys: 129,
+        dia: 79,
+        target: { sysLow: 120, sysHigh: 129, diaLow: 70, diaHigh: 79 },
+      }),
       representative: { sys: 129, dia: 79 },
       oldestAt: new Date("2026-07-01T08:00:00Z"),
       latestAt: new Date("2026-07-27T08:00:00Z"),
@@ -63,7 +72,11 @@ describe("reference-score pillars", () => {
     const result = computeBloodPressurePillar({
       ...live,
       pairCount: 12,
-      gradedScore: 85,
+      graded: gradeBpScore({
+        sys: 129,
+        dia: 79,
+        target: { sysLow: 120, sysHigh: 129, diaLow: 70, diaHigh: 79 },
+      }),
       representative: { sys: 129, dia: 79 },
       oldestAt: new Date("2026-07-01T08:00:00Z"),
       latestAt: new Date("2026-07-27T08:00:00Z"),
diff --git a/src/lib/analytics/score/blood-pressure.ts b/src/lib/analytics/score/blood-pressure.ts
index 0fc5d3534..c1445cec2 100644
--- a/src/lib/analytics/score/blood-pressure.ts
+++ b/src/lib/analytics/score/blood-pressure.ts
@@ -48,7 +48,7 @@ export function computeBloodPressurePillar(
 
   if (
     presentPairs < BLOOD_PRESSURE_MIN_PAIRS ||
-    input.gradedScore == null ||
+    input.graded == null ||
     input.representative == null ||
     input.latestAt == null
   ) {
@@ -81,7 +81,10 @@ export function computeBloodPressurePillar(
       historyDays,
     }),
     value: {
-      score: input.gradedScore,
+      score: input.graded.score,
+      // Same object the score came out of, so the popover can only ever
+      // explain the number beside it.
+      scoreBasis: input.graded.basis,
       observed: {
         value: representativeSys,
         unit: "mmHg",
diff --git a/src/lib/analytics/score/reader.ts b/src/lib/analytics/score/reader.ts
index f02f7a3a9..bf2e6ddfd 100644
--- a/src/lib/analytics/score/reader.ts
+++ b/src/lib/analytics/score/reader.ts
@@ -11,6 +11,7 @@ import { collectBpPairs } from "@/lib/analytics/bp-in-target";
 import {
   gradeBpScoreFromSeries,
   representativeBpFromSeries,
+  type BpGrade,
 } from "@/lib/analytics/bp-grade";
 import type { BpTargets } from "@/lib/analytics/bp-targets";
 import { resolveWeightTargetOverride } from "@/lib/analytics/effective-range";
@@ -86,7 +87,12 @@ interface ActivityRead {
 interface ScoreBpEnvelope {
   path: "rollup" | "live" | undefined;
   last90Days: { pairs: number } | null;
-  gradedScore: number | null;
+  /**
+   * v1.34.5 — score AND basis as one value, straight off the envelope
+   * the route built. Never re-graded here: a second evaluation is how
+   * the popover would end up explaining a different number.
+   */
+  graded: BpGrade | null;
   representative: { sys: number; dia: number } | null;
   last90EarliestAt: Date | null;
   last90LatestAt: Date | null;
@@ -329,7 +335,7 @@ function scoreBpEnvelope(args: {
       envelope: {
         path: args.base.path,
         last90Days: args.base.last90Days,
-        gradedScore: args.base.gradedScore,
+        graded: args.base.graded,
         representative: args.base.representative,
         last90EarliestAt: args.base.last90EarliestAt,
         last90LatestAt: args.base.last90LatestAt,
@@ -352,7 +358,7 @@ function scoreBpEnvelope(args: {
       envelope: {
         path: undefined,
         last90Days: null,
-        gradedScore: null,
+        graded: null,
         representative: null,
         last90EarliestAt: null,
         last90LatestAt: null,
@@ -385,7 +391,7 @@ function scoreBpEnvelope(args: {
     envelope: {
       path: "live",
       last90Days: { pairs: pairs.length },
-      gradedScore: target
+      graded: target
         ? gradeBpScoreFromSeries({
             pairs,
             target,
@@ -492,7 +498,7 @@ function scoreInputsFor(args: {
       readFailed: args.bp.readFailed,
       asOf,
       pairCount: args.bp.last90Days?.pairs ?? 0,
-      gradedScore: args.bp.gradedScore,
+      graded: args.bp.graded,
       representative: args.bp.representative,
       oldestAt: args.bp.last90EarliestAt,
       latestAt: args.bp.last90LatestAt,
diff --git a/src/lib/analytics/score/types.ts b/src/lib/analytics/score/types.ts
index 3d4e9b7ec..086749b95 100644
--- a/src/lib/analytics/score/types.ts
+++ b/src/lib/analytics/score/types.ts
@@ -1,4 +1,5 @@
 import type { BpTargets } from "@/lib/analytics/bp-targets";
+import type { BpGrade, BpScoreBasis } from "@/lib/analytics/bp-grade";
 import type {
   Derived,
   DerivedProvenanceSource,
@@ -56,6 +57,14 @@ export interface PillarValue {
   reference: PillarReference;
   /** Optional user-authored yardstick, displayed beside the scored reference. */
   personalReference?: PillarReference;
+  /**
+   * Which input set this score and by how much, resolved on the server so
+   * no client ever re-derives it. Blood pressure only for now: it is the
+   * one pillar that scores the WORSE of two axes, so the number alone
+   * cannot say which one it came from. Optional by design — a pillar that
+   * has nothing of the kind to say says nothing.
+   */
+  scoreBasis?: BpScoreBasis;
   /** Score points that a weekly move must clear before it is narrated. */
   noiseFloor: number;
   /** Slow markers contribute to level but never to the weekly delta. */
@@ -125,7 +134,11 @@ export interface DomainReadState {
 export interface BloodPressurePillarInput extends DomainReadState {
   asOf: Date;
   pairCount: number;
-  gradedScore: number | null;
+  /**
+   * The graded score together with the basis that produced it — one
+   * value, from one grading run against one set of targets.
+   */
+  graded: BpGrade | null;
   representative: { sys: number; dia: number } | null;
   oldestAt: Date | null;
   latestAt: Date | null;
diff --git a/tests/integration/analytics-health-score.test.ts b/tests/integration/analytics-health-score.test.ts
index f74f69b6e..43a8c0fd3 100644
--- a/tests/integration/analytics-health-score.test.ts
+++ b/tests/integration/analytics-health-score.test.ts
@@ -58,7 +58,17 @@ interface PillarEnvelope {
   result: {
     status: "ok" | "insufficient";
     reason?: string;
-    value?: { score: number; observed: { value: number } };
+    value?: {
+      score: number;
+      observed: { value: number };
+      scoreBasis?: {
+        axis: string;
+        relation: string;
+        offsetMmHg: number;
+        boundaryMmHg: number;
+      };
+      personalReference?: { label: string };
+    };
   };
 }
 
@@ -260,6 +270,84 @@ describe("GET /api/analytics — Health Score (v1.34 reference composite)", () =
     expect(hs.composite.value).toBeUndefined();
   });
 
+  it("delivers the BP score basis to the client from the same run that produced the score", async () => {
+    // v1.34.5 — the popover explains "why 56" from a server-resolved basis.
+    // The basis and the score must come from ONE grading run: the route
+    // deliberately re-runs the BP helper against the CLINICAL band when the
+    // user keeps a personal target, and a basis taken from the other run
+    // would explain a number nobody is looking at.
+    //
+    // This asserts through the REAL route and the REAL JSON response, not a
+    // hand-built payload, because the assembly between the grader and the
+    // client is exactly where a correct field goes missing.
+    const user = await seedSession("hs-bp-basis");
+    const prisma = getPrismaClient();
+    const now = Date.now();
+    const DAY = 24 * 60 * 60 * 1000;
+
+    // A personal band that is WIDER than the clinical one on both axes, so
+    // the two runs disagree about everything the basis carries:
+    //   clinical 129/79 → diastolic binds, 9 mmHg over its 79 ceiling → 55
+    //   personal 135/90 → both axes in band                          → 88
+    await prisma.user.update({
+      where: { id: user.id },
+      data: {
+        thresholdsJson: {
+          BLOOD_PRESSURE_SYS: { min: 120, max: 135 },
+          BLOOD_PRESSURE_DIA: { min: 70, max: 90 },
+        },
+      },
+    });
+    for (let i = 0; i < 20; i++) {
+      const at = new Date(now - i * DAY);
+      await prisma.measurement.create({
+        data: {
+          userId: user.id,
+          type: "BLOOD_PRESSURE_SYS",
+          value: 132,
+          unit: "mmHg",
+          measuredAt: at,
+        },
+      });
+      await prisma.measurement.create({
+        data: {
+          userId: user.id,
+          type: "BLOOD_PRESSURE_DIA",
+          value: 88,
+          unit: "mmHg",
+          measuredAt: at,
+        },
+      });
+    }
+
+    const { GET } = await import("@/app/api/analytics/route");
+    const res = await (GET as (req: Request) => Promise<Response>)(
+      new Request("http://localhost/api/analytics"),
+    );
+    expect(res.status).toBe(200);
+    const env = (await res.json()) as AnalyticsEnvelope;
+    const bp = env.data!.healthScore!.pillars.find(
+      (p) => p.id === "BLOOD_PRESSURE",
+    )!;
+    expect(bp.result.status).toBe("ok");
+
+    // The personal band reached the payload, so the route really did take
+    // the branch that re-grades against the clinical one.
+    expect(bp.result.value!.personalReference!.label).toContain("135");
+
+    // Score AND basis, both from the clinical run: the diastolic axis, nine
+    // over ITS clinical ceiling of 79. Under the personal band the same
+    // readings sit inside it and score 88 — so a basis sourced from that
+    // second run cannot pass here.
+    expect(bp.result.value!.score).toBe(55);
+    expect(bp.result.value!.scoreBasis).toEqual({
+      axis: "diastolic",
+      relation: "above_ceiling",
+      offsetMmHg: 9,
+      boundaryMmHg: 79,
+    });
+  });
+
   it("stays insufficient with only two eligible domains — the three-domain floor", async () => {
     // The composite requires at least three eligible domains, including a
     // measured physiological one, before it grades at all (CORE_MIN_ELIGIBLE