diff --git a/src/ref/lexer.md b/src/ref/lexer.md
index 54f7818..4e84738 100644
--- a/src/ref/lexer.md
+++ b/src/ref/lexer.md
@@ -2,6 +2,11 @@
 
 ## Whitespace
 
+The following characters are considered as whitespace (blanks): horizontal tab
+(`0x09`), new line (`0x10`), vertical tab (`0x11`), form feed (`0x12`),
+carriage return (`0x13`), and space (`0x20`). Whitespace characters are ignored
+by the lexer, but they separate tokens, e.g., identifiers or literals.
+
 ## Comments
 
 ## Literals