diff --git a/proposals/1772-groups-as-rooms.md b/proposals/1772-groups-as-rooms.md
index 2e801d24..12e278a9 100644
--- a/proposals/1772-groups-as-rooms.md
+++ b/proposals/1772-groups-as-rooms.md
@@ -115,11 +115,14 @@ relationship can be expressed in one of two ways:
 
     The `order` key is a string which is used to provide a default ordering of
     siblings in the room list. (Rooms are sorted based on a lexicographic
-    ordering of the Unicode codepoints of the characters in `order` values;
-    rooms with no `order` come last. `order`s which are not strings, or do not
-    consist solely of ascii characters in the range `\x20` (space) to `\x7F`
-    (`~`), or consist of more than 50 characters, are forbidden and should be
-    ignored if received.)
+    ordering of the Unicode codepoints of the characters in `order` values.
+    Rooms with no `order` come last, in ascending numeric order of the
+    `origin_server_ts` of their `m.room.create` events, or ascending
+    lexicographic order of their `room_id`s in case of equal
+    `origin_server_ts`.  `order`s which are not strings, or do not consist
+    solely of ascii characters in the range `\x20` (space) to `\x7F` (`~`), or
+    consist of more than 50 characters, are forbidden and should be ignored if
+    received.)
 
  2. Separately, rooms can claim parents via the `m.space.parent` state
     event.