diff --git a/changelogs/room_versions/newsfragments/1422.clarification b/changelogs/room_versions/newsfragments/1422.clarification
new file mode 100644
index 00000000..efb63c69
--- /dev/null
+++ b/changelogs/room_versions/newsfragments/1422.clarification
@@ -0,0 +1 @@
+Clarify the grammar for room versions.
diff --git a/content/rooms/_index.md b/content/rooms/_index.md
index 190b305e..e75fbb13 100644
--- a/content/rooms/_index.md
+++ b/content/rooms/_index.md
@@ -86,7 +86,7 @@ split-brain situation due to not understanding the new rules.
 
 A room version is defined as a string of characters which MUST NOT
 exceed 32 codepoints in length. Room versions MUST NOT be empty and
-SHOULD contain only the characters `a-z`, `0-9`, `.`, and `-`.
+MUST contain only the characters `a-z`, `0-9`, `.`, and `-`.
 
 Room versions are not intended to be parsed and should be treated as
 opaque identifiers. Room versions consisting only of the characters