diff --git a/docs/grammar/abnf-grammar.txt b/docs/grammar/abnf-grammar.txt
index 2a080affdb..9f326325b8 100644
--- a/docs/grammar/abnf-grammar.txt
+++ b/docs/grammar/abnf-grammar.txt
@@ -59,7 +59,7 @@
 ; usable as last resort in the definiens of a nonterminal.
 
 ; While BNF allows arbitrary terminals,
-; ABNF uses only natural numbers as terminals,
+; ABNF uses only natural numbers (i.e. non-negative integers) as terminals,
 ; and denotes them via:
 ; (i) binary, decimal, or hexadecimal sequences,
 ; e.g. `%b1.11.1010`, `%d1.3.10`, and `%x.1.3.A`