diff --git a/meta/documentation_style.rst b/meta/documentation_style.rst
index 468b2675..0a8da229 100644
--- a/meta/documentation_style.rst
+++ b/meta/documentation_style.rst
@@ -185,3 +185,10 @@ The subheadings which are always deemed as client-affecting are:
 * Redactions
 
 When a new subheading is added, it must be referenced and ordered in this document.
+
+Describing grammar
+~~~~~~~~~~~~~~~~~~
+
+Use  [RFC5234-style ABNF](https://datatracker.ietf.org/doc/html/rfc5234) when describing
+the grammar for something in the spec, such as user IDs or server names. Use lowercase
+and underscore-deliminated element names (`user_id`, not `UserID` or `user-id`).