diff --git a/docs/reference.md b/docs/reference.md
index 11da413..6ce29cf 100644
--- a/docs/reference.md
+++ b/docs/reference.md
@@ -28,6 +28,7 @@ The quick lookup surface: one line and a minimal snippet per construct. For rule
 | [`hierarchy` / `leafOnly`](/spec/entities#hierarchy-leafonly-tree-entities) | tree entities, leaf-only references |
 | [calculated fields](/spec/entities#calculated-fields) | server + UI-evaluated expressions, date helpers, call-outs |
 | [`relations` / `composition`](/spec/relations#relations) | associations and master-detail compositions |
+| [`relations` / `whenMasterDeleted`](/spec/relations#deleting-the-master-whenmasterdeleted) | whether deleting a master deletes the composition children it owns, or is refused while they exist |
 | [`uses`](/spec/relations#multi-model-applications) | reuse entities owned by another intent model |
 | [`related`](/spec/relations#related) | a read-only register of the records referencing this entity, on its own page |
 | [`processes`](/spec/processes#processes) | workflows: user tasks, decisions, waits, boundary timers |
diff --git a/docs/spec/relations.md b/docs/spec/relations.md
index 2da93b8..d5bae91 100644
--- a/docs/spec/relations.md
+++ b/docs/spec/relations.md
@@ -21,6 +21,29 @@ Relation kinds: `oneToMany`, `manyToOne`, `oneToOne`, `manyToMany`. The foreign
 
 Composition is **opt-in** — most required FKs are plain associations, and composition is explicit.
 
+### Deleting the master — `whenMasterDeleted`
+
+Composition is ownership: the child is a detail of its master, with a NOT NULL key because it cannot exist without it. So a delete of the master cannot leave those rows behind — an orphan is invisible in the UI (a detail is reached through its master's page, and the master is gone) while every report, roll-up and aggregate over the child keeps counting it. `whenMasterDeleted` chooses between the two sane outcomes, on the child that declares the composition:
+
+```yaml
+- name: SalesOrderItem
+  relations:
+    # deleted with the order — the default, so the key may be omitted
+    - { name: order, kind: manyToOne, to: SalesOrder, composition: true, whenMasterDeleted: cascade }
+
+- name: SalesOrderCopy
+  relations:
+    # the order cannot be deleted while a copy of it exists
+    - { name: order, kind: manyToOne, to: SalesOrder, composition: true, whenMasterDeleted: refuse }
+```
+
+- **`cascade`** (the default) — the children go with the master, and each child's deletion is a deletion in full: its own composition children with it (a chain of any depth unwinds), its deletion observed by the reactions bound to it, so aggregates and roll-ups over the child relinquish what they counted. The master's deletion and the children's are one atomic unit.
+- **`refuse`** — the master's delete is rejected while any child of that relation exists, naming both entities. Nothing is deleted; the master becomes deletable once the children have been removed deliberately.
+
+::: info Normative
+A conforming generator must, for every composition, either delete the children with the master or reject the master's deletion — the default is `cascade`, whether or not the key is authored, because it is what composition means. Both outcomes must hold for **every** writer that can delete the master, not only for a delete arriving over a generated interface: a reaction, a scheduled write and a cascade from a further master reach the same rows. `whenMasterDeleted` is valid only on a `composition: true` to-one, and only on the entity's first composition (its composition parent); anywhere else, and for any value other than `cascade` or `refuse`, it is an error.
+:::
+
 ### Relation attributes
 
 ```yaml