diff --git a/frontend/src/lib/person/genealogy/layout/familyForest.test.ts b/frontend/src/lib/person/genealogy/layout/familyForest.test.ts
index 33ea1c51..ef086bf4 100644
--- a/frontend/src/lib/person/genealogy/layout/familyForest.test.ts
+++ b/frontend/src/lib/person/genealogy/layout/familyForest.test.ts
@@ -1,8 +1,52 @@
 import { describe, it, expect } from 'vitest';
-import { pickStructuralOwner } from './familyForest';
+import { pickStructuralOwner, buildFamilyForest, type Unit } from './familyForest';
+import type { components } from '$lib/generated/api';
+
+type PersonNodeDTO = components['schemas']['PersonNodeDTO'];
+type RelationshipDTO = components['schemas']['RelationshipDTO'];
 
 type Person = { id: string; birthYear?: number };
 
+function person(id: string, opts: { birthYear?: number; generation?: number } = {}): PersonNodeDTO {
+	const n: PersonNodeDTO = { id, displayName: id, familyMember: true };
+	if (opts.birthYear != null) n.birthYear = opts.birthYear;
+	if (opts.generation != null) n.generation = opts.generation;
+	return n;
+}
+
+function parent(p: string, c: string): RelationshipDTO {
+	return {
+		id: `${p}>${c}`,
+		personId: p,
+		relatedPersonId: c,
+		personDisplayName: '',
+		relatedPersonDisplayName: '',
+		relationType: 'PARENT_OF'
+	};
+}
+
+function spouse(a: string, b: string, fromYear?: number): RelationshipDTO {
+	return {
+		id: `${a}~${b}`,
+		personId: a,
+		relatedPersonId: b,
+		personDisplayName: '',
+		relatedPersonDisplayName: '',
+		relationType: 'SPOUSE_OF',
+		...(fromYear != null ? { fromYear } : {})
+	};
+}
+
+// Find the unit (anywhere in the forest) whose primary id matches.
+function findUnit(roots: Unit[], primaryId: string): Unit | undefined {
+	for (const r of roots) {
+		if (r.id === primaryId) return r;
+		const found = findUnit(r.children, primaryId);
+		if (found) return found;
+	}
+	return undefined;
+}
+
 describe('pickStructuralOwner', () => {
 	const p = (id: string, birthYear?: number): Person => ({ id, birthYear });
 
@@ -21,3 +65,46 @@ describe('pickStructuralOwner', () => {
 		expect(pickStructuralOwner(p('zzz'), p('aaa'))).toBe('aaa');
 	});
 });
+
+describe('buildFamilyForest — loose-spouse absorption', () => {
+	it('absorbs a parentless spouse into the partner run; their child anchors to the couple', () => {
+		// A (founder) ⚭ S (married-in, no parents). Their child C. S has no
+		// ancestor subtree of its own, but C still hangs off the couple.
+		const forest = buildFamilyForest(
+			[
+				person('A', { birthYear: 1900 }),
+				person('S', { birthYear: 1905 }),
+				person('C', { birthYear: 1930 })
+			],
+			[spouse('A', 'S'), parent('A', 'C'), parent('S', 'C')]
+		);
+
+		// One root unit (A), with S absorbed — S is not its own root.
+		expect(forest.roots.map((r) => r.id)).toEqual(['A']);
+		const a = findUnit(forest.roots, 'A')!;
+		expect(a.members).toEqual(['A', 'S']);
+		// The child anchors through the couple unit.
+		expect(a.children.map((u) => u.id)).toEqual(['C']);
+		// S is parentless, so no displaced cross-link.
+		expect(forest.crossLinks).toEqual([]);
+	});
+
+	it('keeps all marriages of a multi-spouse founder in one run (marriage-year order)', () => {
+		// Albert-like: one founder, three parentless wives. All absorbed into the
+		// founder run, ordered by marriage year NULLS LAST then displayName/id.
+		const forest = buildFamilyForest(
+			[
+				person('alb', { birthYear: 1829 }),
+				person('w1925', { birthYear: 1900 }),
+				person('wNull', { birthYear: 1901 }),
+				person('w1910', { birthYear: 1902 })
+			],
+			[spouse('alb', 'w1925', 1925), spouse('alb', 'wNull'), spouse('alb', 'w1910', 1910)]
+		);
+
+		expect(forest.roots.map((r) => r.id)).toEqual(['alb']);
+		const alb = findUnit(forest.roots, 'alb')!;
+		// Founder first, then spouses by marriage year (1910, 1925, null last).
+		expect(alb.members).toEqual(['alb', 'w1910', 'w1925', 'wNull']);
+	});
+});
diff --git a/frontend/src/lib/person/genealogy/layout/familyForest.ts b/frontend/src/lib/person/genealogy/layout/familyForest.ts
index 6feb0a5e..1fc2be4e 100644
--- a/frontend/src/lib/person/genealogy/layout/familyForest.ts
+++ b/frontend/src/lib/person/genealogy/layout/familyForest.ts
@@ -9,6 +9,32 @@
 import type { components } from '$lib/generated/api';
 
 type PersonNodeDTO = components['schemas']['PersonNodeDTO'];
+type RelationshipDTO = components['schemas']['RelationshipDTO'];
+
+/**
+ * A family unit = one bloodline carrier (the primary) plus the spouse(s)
+ * absorbed into its run. `members[0]` is the primary; the rest are spouses in
+ * marriage-year order. `children` are the units anchored by the couple's
+ * offspring (the forest hierarchy).
+ */
+export type Unit = {
+	id: string; // == primary's id; also the tidyTree node id
+	members: string[]; // run order: [primary, ...absorbed spouses]
+	children: Unit[];
+};
+
+/**
+ * A parent→child edge whose child is NOT positioned under that parent (the
+ * child lives in a spouse's run elsewhere). `sameLevel` is true when the
+ * displaced parent can be ordered adjacent to the child's anchor (a short,
+ * solid connector — the intra-family "adjacency" case); false marks a genuine
+ * cross-level link that must render with a distinct dash.
+ */
+export type CrossLink = { parentId: string; childId: string; sameLevel: boolean };
+
+export type FamilyForest = { roots: Unit[]; crossLinks: CrossLink[] };
+
+const ROOT_GROUP = '__ROOT__';
 
 /**
  * Choose the structural owner of a couple: the spouse who keeps the bloodline
@@ -31,4 +57,183 @@ function ownerRank(p: { birthYear?: number | null }): number {
 	return p.birthYear == null ? Number.POSITIVE_INFINITY : p.birthYear;
 }
 
+/**
+ * Build the family forest: assign every person to exactly one unit (a primary,
+ * or a spouse absorbed into a primary's run), wire the parent/child hierarchy,
+ * and record displaced parent edges as cross-links. The packer (tidyTree) and
+ * the orchestrator (buildLayout) consume the result; this module holds all the
+ * genealogy semantics.
+ */
+export function buildFamilyForest(nodes: PersonNodeDTO[], edges: RelationshipDTO[]): FamilyForest {
+	const byId = new Map(nodes.map((n) => [n.id, n]));
+	const allIds = new Set(nodes.map((n) => n.id));
+	const parentToChildren = new Map<string, string[]>();
+	const childToParents = new Map<string, string[]>();
+	const spouses = new Map<string, Set<string>>();
+	const spouseYear = new Map<string, number | undefined>();
+
+	for (const e of edges) {
+		// Unknown-id guard for BOTH edge types: an edge to an id outside the node
+		// list is dropped, never dereferenced into an undefined position.
+		if (!allIds.has(e.personId) || !allIds.has(e.relatedPersonId)) continue;
+		if (e.relationType === 'PARENT_OF') {
+			push(parentToChildren, e.personId, e.relatedPersonId);
+			push(childToParents, e.relatedPersonId, e.personId);
+		} else if (e.relationType === 'SPOUSE_OF') {
+			addToSet(spouses, e.personId, e.relatedPersonId);
+			addToSet(spouses, e.relatedPersonId, e.personId);
+			spouseYear.set(pairKey(e.personId, e.relatedPersonId), e.fromYear ?? undefined);
+		}
+	}
+
+	const hasParents = (id: string) => (childToParents.get(id)?.length ?? 0) > 0;
+
+	// --- Absorption: decide who is absorbed into whose run. ---
+	const absorbedInto = new Map<string, string>();
+	const runOwner = (id: string): string => {
+		let cur = id;
+		const seen = new Set([cur]);
+		while (absorbedInto.has(cur)) {
+			cur = absorbedInto.get(cur)!;
+			if (seen.has(cur)) break; // defensive: never loop on a self-referential chain
+			seen.add(cur);
+		}
+		return cur;
+	};
+	const owners = new Set<string>();
+	const canAbsorb = (x: string) => !absorbedInto.has(x) && !owners.has(x);
+
+	for (const [a, b] of marriagesInOrder(spouses)) {
+		if (runOwner(a) === runOwner(b)) continue;
+		const aP = hasParents(a);
+		const bP = hasParents(b);
+		let owner: string;
+		let absorbed: string;
+		if (aP === bP) {
+			// Both parented (intra-family) or both parentless (dual-loose founders):
+			// the structural owner keeps the bloodline position.
+			owner = pickStructuralOwner(byId.get(a)!, byId.get(b)!);
+			absorbed = owner === a ? b : a;
+		} else if (aP) {
+			owner = a;
+			absorbed = b;
+		} else {
+			owner = b;
+			absorbed = a;
+		}
+		if (canAbsorb(absorbed)) {
+			const o = runOwner(owner);
+			absorbedInto.set(absorbed, o);
+			owners.add(o);
+		} else if (canAbsorb(owner)) {
+			const o = runOwner(absorbed);
+			absorbedInto.set(owner, o);
+			owners.add(o);
+		}
+		// else: both already fixed in other runs — the marriage still draws a
+		// plain spouse line via the connector; no absorption is forced.
+	}
+
+	// --- Unit membership ---
+	const unitOf = new Map<string, string>(); // person → primary id
+	for (const n of nodes) unitOf.set(n.id, runOwner(n.id));
+	const primaries = nodes.map((n) => n.id).filter((id) => unitOf.get(id) === id);
+
+	const absorbedOf = new Map<string, string[]>();
+	for (const k of absorbedInto.keys()) push(absorbedOf, runOwner(k), k);
+
+	const membersOf = new Map<string, string[]>();
+	for (const p of primaries) {
+		const spousesOfP = (absorbedOf.get(p) ?? [])
+			.slice()
+			.sort((x, y) => spouseRun(p, x, y, spouseYear, byId));
+		membersOf.set(p, [p, ...spousesOfP]);
+	}
+
+	// hierarchy parent unit of a primary: the unit of its structural-owner parent
+	// (married co-parents share a unit anyway, so the choice is only material for
+	// the rare unmarried-co-parent case).
+	const hierParentUnit = (primary: string): string | null => {
+		const parents = childToParents.get(primary) ?? [];
+		if (parents.length === 0) return null;
+		let chosen = parents[0];
+		for (const pa of parents.slice(1))
+			chosen = pickStructuralOwner(byId.get(chosen)!, byId.get(pa)!);
+		const u = unitOf.get(chosen)!;
+		return u === primary ? null : u; // guard: a self-parent edge cannot anchor
+	};
+	const groupKey = (primary: string) => hierParentUnit(primary) ?? ROOT_GROUP;
+
+	// --- Build Unit objects + hierarchy ---
+	const unitObj = new Map<string, Unit>();
+	for (const p of primaries) unitObj.set(p, { id: p, members: membersOf.get(p)!, children: [] });
+	const roots: Unit[] = [];
+	for (const p of primaries) {
+		const hp = hierParentUnit(p);
+		if (hp == null) roots.push(unitObj.get(p)!);
+		else unitObj.get(hp)!.children.push(unitObj.get(p)!);
+	}
+
+	// --- Cross-links: displaced parent → absorbed-spouse edges. ---
+	const crossLinks: CrossLink[] = [];
+	for (const k of absorbedInto.keys()) {
+		const owner = runOwner(k);
+		const hu = hierParentUnit(owner);
+		for (const pa of childToParents.get(k) ?? []) {
+			const sameLevel = hu != null && groupKey(unitOf.get(pa)!) === groupKey(hu);
+			crossLinks.push({ parentId: pa, childId: k, sameLevel });
+		}
+	}
+
+	return { roots, crossLinks };
+}
+
+// Spouse-run comparator: marriage year ASC NULLS LAST, then displayName, then id.
+function spouseRun(
+	primary: string,
+	x: string,
+	y: string,
+	spouseYear: Map<string, number | undefined>,
+	byId: Map<string, PersonNodeDTO>
+): number {
+	const yx = spouseYear.get(pairKey(primary, x)) ?? Number.POSITIVE_INFINITY;
+	const yy = spouseYear.get(pairKey(primary, y)) ?? Number.POSITIVE_INFINITY;
+	if (yx !== yy) return yx - yy;
+	const nx = byId.get(x)?.displayName ?? '';
+	const ny = byId.get(y)?.displayName ?? '';
+	return nx.localeCompare(ny) || (x < y ? -1 : 1);
+}
+
+// Unique undirected marriages in a deterministic (pair-key) order.
+function marriagesInOrder(spouses: Map<string, Set<string>>): [string, string][] {
+	const seen = new Set<string>();
+	const out: [string, string][] = [];
+	for (const a of [...spouses.keys()].sort()) {
+		for (const b of [...spouses.get(a)!].sort()) {
+			const k = pairKey(a, b);
+			if (seen.has(k)) continue;
+			seen.add(k);
+			out.push(a < b ? [a, b] : [b, a]);
+		}
+	}
+	out.sort((m1, m2) => (pairKey(m1[0], m1[1]) < pairKey(m2[0], m2[1]) ? -1 : 1));
+	return out;
+}
+
+function push<K, V>(map: Map<K, V[]>, key: K, value: V) {
+	const arr = map.get(key);
+	if (arr) arr.push(value);
+	else map.set(key, [value]);
+}
+
+function addToSet<K, V>(map: Map<K, Set<V>>, key: K, value: V) {
+	const s = map.get(key);
+	if (s) s.add(value);
+	else map.set(key, new Set([value]));
+}
+
+function pairKey(a: string, b: string): string {
+	return a < b ? `${a}|${b}` : `${b}|${a}`;
+}
+
 export type { PersonNodeDTO };