diff --git a/README b/README
index 05eccd0ee..f2189ac96 100644
--- a/README
+++ b/README
@@ -2450,7 +2450,9 @@ can be found at <https://github.com/citation-style-language/styles>.  See also
 Citations go inside square brackets and are separated by semicolons.
 Each citation must have a key, composed of '@' + the citation
 identifier from the database, and may optionally have a prefix,
-a locator, and a suffix.  Here are some examples:
+a locator, and a suffix.  The citation key must begin with a letter
+or `_`, and may contain alphanumerics, `_`, and internal punctuation
+characters (`:.#$%&-+?<>~/`).  Here are some examples:
 
     Blah blah [see @doe99, pp. 33-35; also @smith04, ch. 1].