diff --git a/doc/ref/spec.md b/doc/ref/spec.md
index 22fc29c..2fe4864 100644
--- a/doc/ref/spec.md
+++ b/doc/ref/spec.md
@@ -177,8 +177,9 @@
 ### Identifiers
 
 Identifiers name entities such as fields and aliases.
-An identifier is a sequence of one or more letters (which includes `_`) and digits.
-It may not be `_`.
+An identifier is a sequence of one or more letters (which includes `_` and `$`)
+and digits.
+It may not be `_` or `$`.
 The first character in an identifier must be a letter.
 
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