diff --git a/event-schemas/schema/core-event-schema/state_event.yaml b/event-schemas/schema/core-event-schema/state_event.yaml
index 020e9087..71c4137b 100644
--- a/event-schemas/schema/core-event-schema/state_event.yaml
+++ b/event-schemas/schema/core-event-schema/state_event.yaml
@@ -11,7 +11,11 @@ properties:
   state_key:
     description: A unique key which defines the overwriting semantics for this piece
       of room state. This value is often a zero-length string. The presence of this
-      key makes this event a State Event. The key MUST NOT start with '_'.
+      key makes this event a State Event. 
+
+      State keys starting with an ``@`` are reserved for referencing user IDs, such
+      as room members. With the exception of a few events, state events set with a
+      given user's ID as the state key MUST only be set by that user.
     type: string
 required:
 - state_key
diff --git a/event-schemas/schema/m.room.member b/event-schemas/schema/m.room.member
index 5fb5356d..de14644d 100644
--- a/event-schemas/schema/m.room.member
+++ b/event-schemas/schema/m.room.member
@@ -105,7 +105,10 @@ properties:
     title: EventContent
     type: object
   state_key:
-    description: The ``user_id`` this membership event relates to.
+    description: |-
+      The ``user_id`` this membership event relates to. In all cases except for when ``membership`` is
+      ``join``, the user ID sending the event does not need to match the user ID in the ``state_key``,
+      unlike other events. Regular authorisation rules still apply.
     type: string
   type:
     enum: