diff --git a/versions/1.2.md b/versions/1.2.md
index f321557..2bd6c1c 100644
--- a/versions/1.2.md
+++ b/versions/1.2.md
@@ -492,10 +492,21 @@ A guard compares a keyed [aggregate](#aggregates--keyed-cross-entity-totals) aga
 
 `immutableWhen` requires a `function: EntityStatus` relation; `immutable: true` needs none and is mutually exclusive with it. System / workflow writes stay possible — corrections to an immutable record are flow-generated reversals, never edits.
 
+An entity's immutability also covers its **composition children**, which declare none of their own and whose writes maintain the master's derived values — see [`locksWithMaster`](#lockswithmaster--a-child-collection-that-outlives-its-masters-lock) for the collection that must outlive the lock.
+
+> **Normative.**
+> A generator MUST refuse a user create, update or delete of a composition child whose master is
+> currently immutable, unless that child declares `locksWithMaster: false`. The refusal MUST cover
+> every user surface it generates, not only the affordances it renders: a child write that
+> maintains the master's derived values — a line that resums the document's totals — reaches
+> exactly what the lock protects, and permitting it undoes the lock through a different door. It
+> MUST NOT extend to system / workflow writes, which are what corrects an immutable record.
+
 ### locksWithMaster — a child collection that outlives its master's lock
 
-An entity's immutability covers **that entity**. A composition child is a different entity, so a
-master that locks says nothing about whether its child collections should:
+An entity's immutability covers that entity **and the collections composed into it**. For some
+children that default is wrong — a master that freezes its own content says nothing about a
+collection recording what happens to the document afterwards:
 
 ```yaml
 - name: Invoice
@@ -512,7 +523,8 @@ different lifecycles on the same document.
 
 > **Normative.**
 > `locksWithMaster` defaults to **true**, so a child that says nothing keeps freezing with its
-> master.
+> master — in the affordances a generator renders for that collection AND in the writes it accepts
+> for it (see [immutability](#immutablewhen--immutable--user-write-immutability)).
 > A generator MUST NOT extend a master's user-write immutability to a child collection declared
 > `locksWithMaster: false` — including the affordances it renders for that collection, not merely
 > the writes it accepts. A read-only rendering that the server would have permitted is the same