diff --git a/grammars/README.md b/grammars/README.md
index a63198b5ae..adf089af64 100644
--- a/grammars/README.md
+++ b/grammars/README.md
@@ -36,7 +36,7 @@ castle ::= ...
 
 ## Non-Terminals and Terminals
 
-Non-terminal symbols (rule names) stand for a pattern of terminals and other non-terminals. They are required to be a dashed lowercase word, like `move`, `castle`, or `check-mate`.
+Non-terminal symbols (rule names) stand for a pattern of terminals and other non-terminals. Rule names can be composed of letters (both uppercase and lowercase), numbers, dashes, and underscores. For example: `move`, `castle`, `check-mate`, `dataType`, `UPPER-CASE`, or `rule_123` are all valid non-terminal names.
 
 Terminals are actual characters ([code points](https://en.wikipedia.org/wiki/Code_point)). They can be specified as a sequence like `"1"` or `"O-O"` or as ranges like `[1-9]` or `[NBKQR]`.