diff --git a/src/data/hours/matins-psalmody-overrides/st-lawrence.yml b/src/data/hours/matins-psalmody-overrides/st-lawrence.yml
new file mode 100644
index 0000000..6d16707
--- /dev/null
+++ b/src/data/hours/matins-psalmody-overrides/st-lawrence.yml
@@ -0,0 +1,118 @@
+# Verified against Divinum Officium (Monastic Tridentinum 1617), live
+# prayMatutinum query (2026-08-17, St. Lawrence's own octave closing day
+# -- "In Octava S. Laurentii Martyris ~ Duplex"). This is the live-
+# verified proof for the Matins-psalmody-override mechanism
+# (hours/matins-psalmody-overrides.ts) -- the general mechanism gap this
+# closes: a Duplex+ weekday feast previously fell through to reusing the
+# literal *Sunday* psalmody (data/hours/matins-sunday-antiphons.yml, Ps
+# 20-31) unconditionally, which is wrong -- a Duplex+ weekday feast has
+# its own real, different psalmody (confirmed against the live engine
+# directly), not the Sunday's.
+#
+# Cross-checked against a second Duplex saint (St. Ignatius of Antioch,
+# Common of a Martyr-Bishop, no proper content of his own, live-queried
+# 2029-02-01): Nocturn 1 is *identical* (Ps 1,2,4,5,8,10); Nocturn 2 is
+# nearly identical (Ps 14,20,23,63,91 match -- only one slot differs,
+# Lawrence has 16 where Ignatius has 64). So the psalm *numbers* are a
+# shared per-Common-category pool, not unique to Lawrence -- only the
+# antiphon *text* is proper to him (each one narrates an episode of his
+# own martyrdom, e.g. "Quo progréderis sine fílio, pater?"). This file is
+# keyed by his own saint id (not yet a Common-category id) since only his
+# own proper antiphons have been authored so far -- see this module's own
+# doc comment for what extending this to a real Common-category key would
+# need.
+#
+# Nocturn 3's first canticle spans two Sirach chapters under one heading
+# in the source ("Eccli 14:22;15:3-4;15:6") -- kept as one canticle with
+# two refs (matins.ts's own `canticles: { refs: [...] }[]` shape), not
+# split into two, matching how the live engine presents it. New scripture
+# chapters authored for this: data/scripture/{sir-14,sir-15,jer-17,sir-31}.yml
+# (all sparse, only the cited verses -- same convention as sir-36.yml's
+# own Sunday-canticle transcription).
+id: st-lawrence
+nocturn1:
+  groups:
+    - psalms: [1]
+      antiphon:
+        la: "Quo progréderis * sine fílio, pater? quo, sacérdos sancte, sine minístro próperas?"
+        en: "Father, whither goest thou * without thy son? Holy Priest, dost thou fare hence without a Deacon?"
+    - psalms: [2]
+      antiphon:
+        la: "Noli me derelínquere, * pater sancte, quia thesáuros tuos jam expéndi, quos tradidísti mihi."
+        en: "Leave me not, holy father, * for I have spent already thy treasures, which thou gavest unto me."
+    - psalms: [4]
+      antiphon:
+        la: "Non ego te désero, * fili, neque derelínquo; sed majóra tibi debéntur pro Christi fide certámina."
+        en: "I leave thee not, my son, neither forsake thee; * there awaiteth thee for Christ's truth a sterner wrestling than mine."
+    - psalms: [5]
+      antiphon:
+        la: "Beátus Lauréntius * orábat, dicens: Dómine Jesu Christe, Deus de Deo, miserére mihi servo tuo."
+        en: "The blessed Lawrence prayed * and said: O Lord Jesus Christ, God of God, take pity upon me thy servant."
+    - psalms: [8]
+      antiphon:
+        la: "Dixit Románus * ad beátum Lauréntium: Vídeo ante te júvenem pulchérrimum, festína me baptizáre."
+        en: "Romanus said * to Blessed Lawrence: I see in front of thee a very well-favoured young man; make haste to baptize me."
+    - psalms: [10]
+      antiphon:
+        la: "Beátus Lauréntius * dixit: Mea nox obscúrum non habet, sed ómnia in luce claréscunt."
+        en: "The blessed Lawrence said: The darkness is no darkness to me, * but the night is all as clear as morning, that shineth more and more unto the perfect day."
+  versicle:
+    v:
+      la: "Glória et honóre coronásti eum, Dómine."
+      en: "Thou hast crowned him with glory and honour, O Lord."
+    r:
+      la: "Et constituísti eum super ópera mánuum tuárum."
+      en: "And madest him to have dominion over the works of thy hands."
+nocturn2:
+  groups:
+    - psalms: [14]
+      antiphon:
+        la: "Strinxérunt * córporis membra pósita super cratículam: subiciéntibus prunas insúltat Levíta Christi."
+        en: "They laid him upon the grating, and stretched out his limbs; * Christ's Levite held in derision them that brought the live coals."
+    - psalms: [16]
+      antiphon:
+        la: "Igne me examinásti, * et non est invénta in me iníquitas."
+        en: "Thou hast tried me with fire, * and found no wickedness in me."
+    - psalms: [20]
+      antiphon:
+        la: "Interrogátus * te Dóminum conféssus sum, assátus grátias ago."
+        en: "When I was asked, I acknowledged thee to be the Lord; now that I am roasted, I give thee thanks."
+    - psalms: [23]
+      antiphon:
+        la: "Beátus Lauréntius * clamávit et dixit: Deum meum colo, et illi solo sérvio."
+        en: "Blessed Lawrence * cried out and said: I care for my God, and serve him alone."
+    - psalms: [63]
+      antiphon:
+        la: "Dispérsit * dedit paupéribus: justítia ejus manet in séculum sǽculi."
+        en: "He hath dispersed and gave to the poor; * his justice shall remain in ages of ages."
+    - psalms: [91]
+      antiphon:
+        la: "Protéctor tuus * ego sum noli timére: si transíeris per ignem, flamma non nocébit tibi, et odor ignis non erit in te."
+        en: "I am thy protector, do not fear; * if you pass through fire, the flame will not harm thee, and the scent of fire will not be upon thee."
+  versicle:
+    v:
+      la: "Posuísti, Dómine, super caput ejus."
+      en: "O Lord, Thou hast set a crown of precious stones."
+    r:
+      la: "Corónam de lápide pretióso."
+      en: "Upon his head."
+nocturn3:
+  antiphon:
+    la: "Nisi granum fruménti * cadens, in terram mórtuum fúerit, ipsum solum manet."
+    en: "Unless the grain of wheat * falls to the ground and dies, it only remains as such."
+  canticles:
+    - refs:
+        - { book: sir, chapter: 14, verses: "22" }
+        - { book: sir, chapter: 15, verses: "3-4" }
+        - { book: sir, chapter: 15, verses: "6" }
+    - refs:
+        - { book: jer, chapter: 17, verses: "7-8" }
+    - refs:
+        - { book: sir, chapter: 31, verses: "8-11" }
+  versicle:
+    v:
+      la: "Magna est glória ejus in salutári tuo."
+      en: "His glory is great in thy salvation."
+    r:
+      la: "Glóriam et magnum decórem impónes super eum."
+      en: "Honour and great majesty shalt Thou lay upon him."
diff --git a/src/data/hours/matins-sunday-antiphons.yml b/src/data/hours/matins-sunday-antiphons.yml
index 57508c9..1094747 100644
--- a/src/data/hours/matins-sunday-antiphons.yml
+++ b/src/data/hours/matins-sunday-antiphons.yml
@@ -62,9 +62,9 @@ nocturn2:
 
 nocturn3:
   canticles:
-    - { book: isa, chapter: 33, verses: "2-10" }
-    - { book: isa, chapter: 33, verses: "13-18" }
-    - { book: sir, chapter: 36, verses: "14-19" }
+    - refs: [{ book: isa, chapter: 33, verses: "2-10" }]
+    - refs: [{ book: isa, chapter: 33, verses: "13-18" }]
+    - refs: [{ book: sir, chapter: 36, verses: "14-19" }]
   antiphon:
     la: "Allelúja, allelúja, allelúja."
     en: "Alleluia, alleluia, alleluia."
diff --git a/src/data/scripture/jer-17.yml b/src/data/scripture/jer-17.yml
new file mode 100644
index 0000000..e366615
--- /dev/null
+++ b/src/data/scripture/jer-17.yml
@@ -0,0 +1,26 @@
+# Jeremiah 17:7-8, transcribed from a live Divinum Officium query
+# (Monastic Tridentinum 1617, command=prayMatutinum, 2026-08-17 — St.
+# Lawrence's own octave closing day, Matins Nocturn III's second
+# canticle). Verse 8 is split into two half-verses by the source itself
+# (each half its own antiphon-verse line); transcribed as two separate
+# entries under the same verse number, matching how the source renders
+# it, not merged into one. Only 7-8 are authored; the rest of the
+# chapter is unauthored.
+book: jer
+chapter: 17
+verses:
+  - n: 7
+    text:
+      la: "Benedíctus vir qui confídit in Dómino, * et erit Dóminus fidúcia ejus."
+      en: "Blessed be the man that trusteth in the Lord, * and the Lord shall be his confidence."
+    status: { la: verified, en: verified }
+  - n: 8
+    text:
+      la: "Et erit quasi lignum quod transplantátur super aquas, quod ad humórem mittit radíces suas, * et non timébit cum vénerit æstus."
+      en: "And he shall be as a tree that is planted by the waters, that spreadeth out its roots towards moisture: * and it shall not fear when the heat cometh."
+    status: { la: verified, en: verified }
+  - n: 8
+    text:
+      la: "Et erit fólium ejus víride, et in témpore siccitátis non erit sollícitum, * nec aliquándo désinet fácere fructum."
+      en: "And the leaf thereof shall be green, and in the time of drought it shall not be solicitous, * neither shall it cease at any time to bring forth fruit."
+    status: { la: verified, en: verified }
diff --git a/src/data/scripture/sir-14.yml b/src/data/scripture/sir-14.yml
new file mode 100644
index 0000000..a7fd5c7
--- /dev/null
+++ b/src/data/scripture/sir-14.yml
@@ -0,0 +1,14 @@
+# Sirach (Ecclesiasticus) 14:22, transcribed from a live Divinum Officium
+# query (Monastic Tridentinum 1617, command=prayMatutinum, 2026-08-17 —
+# St. Lawrence's own octave closing day, Matins Nocturn III's first
+# canticle, "Eccli 14:22;15:3-4;15:6" — see
+# data/hours/matins-psalmody-overrides/st-lawrence.yml). Only this verse
+# is authored; the rest of the chapter is unauthored.
+book: sir
+chapter: 14
+verses:
+  - n: 22
+    text:
+      la: "Beátus vir qui in sapiéntia morábitur, et qui in justítia sua meditábitur, * et in sensu cogitábit circumspectiónem Dei."
+      en: "Blessed is the man that shall continue in wisdom, and that shall meditate in his justice, * and in his mind shall think of the all-seeing eye of God."
+    status: { la: verified, en: verified }
diff --git a/src/data/scripture/sir-15.yml b/src/data/scripture/sir-15.yml
new file mode 100644
index 0000000..62a423e
--- /dev/null
+++ b/src/data/scripture/sir-15.yml
@@ -0,0 +1,25 @@
+# Sirach (Ecclesiasticus) 15:3-4,6, transcribed from a live Divinum
+# Officium query (Monastic Tridentinum 1617, command=prayMatutinum,
+# 2026-08-17 — St. Lawrence's own octave closing day, Matins Nocturn
+# III's first canticle, "Eccli 14:22;15:3-4;15:6" — see
+# data/hours/matins-psalmody-overrides/st-lawrence.yml). Verse 5 is
+# skipped by the citation itself, not an authoring gap; only 3-4 and 6
+# are authored, the rest of the chapter is unauthored.
+book: sir
+chapter: 15
+verses:
+  - n: 3
+    text:
+      la: "Cibábit illum pane vitæ et intelléctus, * et aqua sapiéntiæ salutáris potábit illum:"
+      en: "With the bread of life and understanding, * she shall feed him, and give him the water of wholesome wisdom to drink."
+    status: { la: verified, en: verified }
+  - n: 4
+    text:
+      la: "Et firmábitur in illo, et non flectétur: et continébit illum, et non confundétur: * et exaltábit illum apud próximos suos."
+      en: "And she shall be made strong in him, and he shall not be moved: And she shall hold him fast, and he shall not be confounded: * and she shall exalt him among his neighbours."
+    status: { la: verified, en: verified }
+  - n: 6
+    text:
+      la: "Et nómine ætérno hæreditábit illum * Dóminus Deus noster."
+      en: "And shall cause him to inherit an everlasting name * the Lord our God."
+    status: { la: verified, en: verified }
diff --git a/src/data/scripture/sir-31.yml b/src/data/scripture/sir-31.yml
new file mode 100644
index 0000000..ba17222
--- /dev/null
+++ b/src/data/scripture/sir-31.yml
@@ -0,0 +1,35 @@
+# Sirach (Ecclesiasticus) 31:8-11, transcribed from a live Divinum
+# Officium query (Monastic Tridentinum 1617, command=prayMatutinum,
+# 2026-08-17 — St. Lawrence's own octave closing day, Matins Nocturn
+# III's third canticle). Verse 10 is split into two half-verses by the
+# source itself, transcribed as two separate entries under the same
+# verse number, matching how the source renders it. Only 8-11 are
+# authored; the rest of the chapter is unauthored.
+book: sir
+chapter: 31
+verses:
+  - n: 8
+    text:
+      la: "Beátus vir, qui invéntus est sine mácula, et qui post aurum non ábiit, * nec sperávit in pecúnia et thesáuris."
+      en: "Blessed is the man that is found without blemish: and that hath not gone after gold, * nor put his trust in money nor in treasures."
+    status: { la: verified, en: verified }
+  - n: 9
+    text:
+      la: "Quis est hic, et laudábimus eum? * fecit enim mirabília in vita sua."
+      en: "Who is he, and we will praise him? * for he hath done wonderful things in his life."
+    status: { la: verified, en: verified }
+  - n: 10
+    text:
+      la: "Qui probátus est in illo, et perféctus est, * erit illi glória ætérna:"
+      en: "Who hath been tried thereby, and made perfect, * he shall have glory everlasting."
+    status: { la: verified, en: verified }
+  - n: 10
+    text:
+      la: "Qui pótuit tránsgredi, et non est transgréssus; * fácere mala, et non fecit."
+      en: "He that could have transgressed, and hath not transgressed: * and could do evil things, and hath not done them."
+    status: { la: verified, en: verified }
+  - n: 11
+    text:
+      la: "Ideo stabilíta sunt bona illíus in Dómino, * et eleemósynas illíus enarrábit omnis ecclésia sanctórum."
+      en: "Therefore are his goods established in the Lord, * and all the church of the saints shall declare his alms."
+    status: { la: verified, en: verified }
diff --git a/src/hours/matins-psalmody-overrides.ts b/src/hours/matins-psalmody-overrides.ts
new file mode 100644
index 0000000..9552c04
--- /dev/null
+++ b/src/hours/matins-psalmody-overrides.ts
@@ -0,0 +1,61 @@
+/** Matins' own analogue of hours/lauds-psalmody-overrides.ts, but keyed
+ * by Common category rather than per-saint: live-verified (see
+ * data/hours/matins-psalmody-overrides/st-lawrence.yml's own header)
+ * that the psalm *numbers* for Nocturns 1-2 are shared across saints of
+ * the same Common category (a plain-Duplex Martyr and a plain-Duplex
+ * Martyr-Bishop share all but one of the same twelve psalm numbers) —
+ * only the antiphon text varies, proper to the individual saint when
+ * authored, generic Common-of-* text otherwise. Deliberately the
+ * opposite indexing choice from Lauds' own per-feast override (see that
+ * file's own doc comment) — Lauds' 5-antiphon pool is shared across
+ * every Duplex-majus+ feast regardless of category, so per-feast is the
+ * natural key there; Matins' pool varies *by* category, so per-category
+ * is the natural key here.
+ *
+ * `id` is looked up against `hours/resolve-common.ts`'s
+ * `resolveOfficeWinner(day).id` when it's sanctoral, exactly like every
+ * other per-feast/per-category content lookup in this app — but the
+ * `id` values populated so far are individual saint ids (St. Lawrence
+ * proper), not Common-category ids, since only the live-verified proof
+ * has been authored; extending this to a real Common-category key
+ * (`common-of-a-martyr`, etc., matching `SaintRecord.common`) with
+ * per-saint antiphon overrides layered on top is the deferred bulk-
+ * content work (see TODO.md) — not built yet, so a saint without their
+ * own entry here falls back to the plain ferial weekday table
+ * (matins.ts's own fallback), not to some other saint's proper
+ * antiphons. */
+export interface MatinsPsalmodyGroup {
+  psalms: number[];
+  antiphon: Partial<Record<string, string>>;
+}
+export interface ScriptureCanticleRef {
+  book: string;
+  chapter: number;
+  verses?: string;
+}
+export interface MatinsPsalmodyNocturn {
+  groups?: MatinsPsalmodyGroup[];
+  canticles?: { refs: ScriptureCanticleRef[] }[];
+  antiphon?: Partial<Record<string, string>>;
+  versicle: { v: Partial<Record<string, string>>; r: Partial<Record<string, string>> };
+}
+export interface MatinsPsalmodyOverride {
+  id: string;
+  nocturn1: MatinsPsalmodyNocturn;
+  nocturn2: MatinsPsalmodyNocturn;
+  nocturn3: MatinsPsalmodyNocturn;
+}
+
+const modules = import.meta.glob<{ default: MatinsPsalmodyOverride }>(
+  '../data/hours/matins-psalmody-overrides/*.yml',
+  { eager: true },
+);
+
+const overrides = new Map<string, MatinsPsalmodyOverride>();
+for (const mod of Object.values(modules)) {
+  overrides.set(mod.default.id, mod.default);
+}
+
+export function getMatinsPsalmodyOverride(id: string | undefined): MatinsPsalmodyOverride | undefined {
+  return id ? overrides.get(id) : undefined;
+}
diff --git a/src/hours/matins.ts b/src/hours/matins.ts
index 3106e48..47a3572 100644
--- a/src/hours/matins.ts
+++ b/src/hours/matins.ts
@@ -69,6 +69,7 @@ import { resolveCommon, getDayCollect, resolveOfficeWinner, verifiedText, splitN
 import { getBiblePlanReadings } from '../propers/bible-plan';
 import { getNocturnReadings, type NocturnReading } from '../propers/nocturn-readings';
 import { getOctaveReading } from '../propers/octave-readings';
+import { getMatinsPsalmodyOverride } from './matins-psalmody-overrides';
 import matinsSundayAntiphonsData from '../data/hours/matins-sunday-antiphons.yml';
 
 type BilingualText = Partial<Record<string, string>>;
@@ -76,9 +77,20 @@ interface SundayGroup {
   psalms: number[];
   antiphon: BilingualText;
 }
+interface ScriptureRef {
+  book: string;
+  chapter: number;
+  verses?: string;
+}
 interface SundayNocturn {
   groups?: SundayGroup[];
-  canticles?: { book: string; chapter: number; verses?: string }[];
+  // Usually one ref per canticle; more than one when the source cites a
+  // single canticle across a chapter boundary (e.g. St. Lawrence's own
+  // Nocturn 3, "Eccli 14:22;15:3-4;15:6" — one canticle, two Sirach
+  // chapters) — concatenated in order, not rendered as separate
+  // canticles, matching how the source itself presents it as one entry
+  // under one heading.
+  canticles?: { refs: ScriptureRef[] }[];
   antiphon?: BilingualText;
   versicle: { v: BilingualText; r: BilingualText };
 }
@@ -144,14 +156,14 @@ function sundayCanticleNocturn(group: SundayNocturn, day: LiturgicalDay): Resolv
   const { incipit, full } = splitNamedAntiphon(verifiedText(group.antiphon ?? {}));
   const opening = isDoubleOrHigher(resolveOfficeWinner(day)) ? full : incipit;
   const parts: ResolvedPart[] = (group.canticles ?? []).map((c, i) => {
-    const verses = getScriptureVerses(c.book, c.chapter, c.verses);
+    const verses = c.refs.flatMap((ref) => getScriptureVerses(ref.book, ref.chapter, ref.verses));
     const text: BilingualText = {
       la: verses.map((v) => v.text.la).filter(Boolean).join(' '),
       en: verses.map((v) => v.text.en).filter(Boolean).join(' '),
     };
     return {
       kind: 'canticle' as const,
-      canticleId: `${c.book}-${c.chapter}${c.verses ? `-${c.verses}` : ''}`,
+      canticleId: c.refs.map((ref) => `${ref.book}-${ref.chapter}${ref.verses ? `-${ref.verses}` : ''}`).join('_'),
       text: verifiedText(text),
       antiphon: i === 0 ? opening : undefined,
     };
@@ -165,6 +177,20 @@ function ferialPsalmody(day: LiturgicalDay): ResolvedPart[] {
   return psalmRefParts(getPsalmsFor('matins', day.weekday));
 }
 
+/** Fallback for a Duplex+ weekday feast with no matins-psalmody-overrides
+ * entry authored for its winner yet (most of them, until the bulk-content
+ * pass — see hours/matins-psalmody-overrides.ts's own doc comment): the
+ * plain ferial weekday table, chunked into 3 nocturns instead of 1 (no
+ * antiphons, no canticles — that content doesn't exist for the ferial
+ * table at all, unlike a real per-category override). Direct instruction:
+ * never blank, but honestly not that saint's own real proper psalmody
+ * until it's authored. */
+function ferialPsalmodyThreeNocturns(day: LiturgicalDay): [ResolvedPart[], ResolvedPart[], ResolvedPart[]] {
+  const psalms = ferialPsalmody(day);
+  const size = Math.ceil(psalms.length / 3);
+  return [psalms.slice(0, size), psalms.slice(size, size * 2), psalms.slice(size * 2)];
+}
+
 /** Every id whose own patristic/hagiographic/Gospel content should be
  * gathered for `day` — the office winner (if sanctoral), every
  * commemorated saint (a transferred-in feast already appears as `day.winner`
@@ -279,7 +305,7 @@ export function resolveOrdo(date: string): ResolvedOrdo {
     { kind: 'hymn', text: resolveCommon('matins-hymn-ferial') },
   ];
 
-  if (threeNocturns) {
+  if (threeNocturns && day.weekday === 'sunday') {
     parts.push(...sundayPsalmNocturn(sundayAntiphons.nocturn1, day));
     parts.push(...(nocturn1Readings ?? []));
     parts.push(...sundayPsalmNocturn(sundayAntiphons.nocturn2, day));
@@ -287,6 +313,30 @@ export function resolveOrdo(date: string): ResolvedOrdo {
     parts.push(...sundayCanticleNocturn(sundayAntiphons.nocturn3, day));
     parts.push(...(nocturn3Readings ?? []));
     parts.push({ kind: 'te-deum', text: resolveCommon('te-deum') });
+  } else if (threeNocturns) {
+    // A Duplex+ weekday feast — genuinely different psalmody from a real
+    // Sunday's (see hours/matins-psalmody-overrides.ts's own doc
+    // comment), not the same content reused. Uses that saint's own
+    // Common-category override when authored, falling back to the plain
+    // ferial weekday table (redistributed into 3 nocturns) otherwise.
+    const override = winner.kind === 'sanctoral' ? getMatinsPsalmodyOverride(winner.id) : undefined;
+    if (override) {
+      parts.push(...sundayPsalmNocturn(override.nocturn1, day));
+      parts.push(...(nocturn1Readings ?? []));
+      parts.push(...sundayPsalmNocturn(override.nocturn2, day));
+      parts.push(...(nocturn2Readings ?? []));
+      parts.push(...sundayCanticleNocturn(override.nocturn3, day));
+      parts.push(...(nocturn3Readings ?? []));
+    } else {
+      const [n1, n2, n3] = ferialPsalmodyThreeNocturns(day);
+      parts.push(...n1);
+      parts.push(...(nocturn1Readings ?? []));
+      parts.push(...n2);
+      parts.push(...(nocturn2Readings ?? []));
+      parts.push(...n3);
+      parts.push(...(nocturn3Readings ?? []));
+    }
+    parts.push({ kind: 'te-deum', text: resolveCommon('te-deum') });
   } else {
     parts.push(...ferialPsalmody(day));
     parts.push(...(nocturn1Readings ?? []));
diff --git a/tests/hours/matins.test.ts b/tests/hours/matins.test.ts
index 0723da9..f244f0c 100644
--- a/tests/hours/matins.test.ts
+++ b/tests/hours/matins.test.ts
@@ -1,5 +1,6 @@
 import { describe, expect, it } from 'vitest';
 import { resolveOrdo } from '../../src/hours';
+import { getPsalmsFor } from '../../src/psalter/distribution';
 
 // Two clean proof dates, one per branch — see hours/matins.ts's own header
 // and TODO.md for why this is a mechanism build with a small content slice,
@@ -120,3 +121,65 @@ describe('resolveOrdo("matins", ...) Sunday (3-nocturn) branch', () => {
     expect(prayerIndex).toBeGreaterThan(teDeumIndex);
   });
 });
+
+describe('resolveOrdo("matins", ...) Duplex+ weekday-feast (3-nocturn, non-Sunday) branch', () => {
+  // 2026-08-17 -- St. Lawrence's own octave closing day ("In Octava S.
+  // Laurentii Martyris ~ Duplex"), live-verified directly against the
+  // reference engine (Monastic Tridentinum 1617, command=prayMatutinum).
+  // The mechanism bug this proves fixed: a Duplex+ weekday feast used to
+  // fall through to reusing the literal *Sunday* psalmody (Ps 20-31)
+  // unconditionally -- confirmed wrong, since a real Duplex+ weekday
+  // feast has its own genuinely different psalmody (see
+  // hours/matins-psalmody-overrides.ts and its own st-lawrence.yml proof
+  // data for the full sourcing detail).
+  const ordo = resolveOrdo('matins', '2026-08-17');
+
+  it("uses St. Lawrence's own proper psalmody, not the Sunday scheme, across 3 nocturns plus a Te Deum", () => {
+    const psalms = ordo.parts.filter((p) => p.kind === 'psalm').map((p) => (p as { psalmNumber: number }).psalmNumber);
+    // Ps 3 + 94 (opening/invitatory), then his own Nocturn 1 (1,2,4,5,8,10)
+    // and Nocturn 2 (14,16,20,23,63,91) -- confirmed live, not the Sunday
+    // scheme's Ps 20-31 at all.
+    expect(psalms).toEqual([3, 94, 1, 2, 4, 5, 8, 10, 14, 16, 20, 23, 63, 91]);
+    expect(ordo.parts.some((p) => p.kind === 'te-deum')).toBe(true);
+  });
+
+  it("every Nocturn 1/2 psalm carries St. Lawrence's own proper antiphon, verified in both languages", () => {
+    const psalmParts = ordo.parts.filter((p) => p.kind === 'psalm') as { psalmNumber: number; antiphon?: { status: Record<string, string> } }[];
+    // Skip Ps 3 (opening, no antiphon) and Ps 94 (the Invitatory, its own
+    // separate antiphon convention) -- the remaining 12 are his own.
+    const nocturnPsalms = psalmParts.filter((p) => ![3, 94].includes(p.psalmNumber));
+    expect(nocturnPsalms).toHaveLength(12);
+    for (const p of nocturnPsalms) {
+      expect(p.antiphon?.status.la).toBe('verified');
+      expect(p.antiphon?.status.en).toBe('verified');
+    }
+  });
+
+  it('has 3 real OT canticles in Nocturn 3, verified in both languages, sourced from the newly-authored Sirach/Jeremiah chapters', () => {
+    const canticles = ordo.parts.filter((p) => p.kind === 'canticle') as { canticleId: string; text: { status: Record<string, string> } }[];
+    expect(canticles).toHaveLength(3);
+    // The first canticle spans two Sirach chapters under one heading in
+    // the source -- kept as one canticle with two refs, not split.
+    expect(canticles[0]?.canticleId).toBe('sir-14-22_sir-15-3-4_sir-15-6');
+    for (const c of canticles) {
+      expect(c.text.status.la).toBe('verified');
+      expect(c.text.status.en).toBe('verified');
+    }
+  });
+
+  it("falls back to the plain ferial weekday table, redistributed into 3 nocturns, for a Duplex+ weekday winner with no override authored", () => {
+    // St. Ignatius of Antioch (Duplex, Common of a Martyr-Bishop, no
+    // matins-psalmody-overrides entry authored) -- Feb 1 2029, confirmed
+    // clean (no Sunday collision) per his own saint-file comment.
+    const fallback = resolveOrdo('matins', '2029-02-01');
+    const psalms = fallback.parts.filter((p) => p.kind === 'psalm').map((p) => (p as { psalmNumber: number }).psalmNumber);
+    // Ps 3 + 94, then Thursday's own 9-psalm ferial table (psalter-
+    // distribution.yml), not St. Lawrence's proper psalmody and not the
+    // Sunday scheme either.
+    expect(psalms[0]).toBe(3);
+    expect(psalms[1]).toBe(94);
+    expect(psalms.slice(2)).toEqual(getPsalmsFor('matins', 'thursday').map((r) => r.number));
+    expect(fallback.parts.some((p) => p.kind === 'te-deum')).toBe(true);
+    expect(fallback.parts.some((p) => p.kind === 'canticle')).toBe(false);
+  });
+});