diff --git a/texk/web2c/pdftexdir/ChangeLog b/texk/web2c/pdftexdir/ChangeLog
index f270c615f1..6dfdcabd84 100644
--- a/texk/web2c/pdftexdir/ChangeLog
+++ b/texk/web2c/pdftexdir/ChangeLog
@@ -1,3 +1,9 @@
+2023-09-01  Karl Berry  <karl@freefriends.org>
+
+	* ptexmac.h (cmp_return): a few words of documentation about
+	the seemingly strange lack of handling the a==b case.
+	Suggested by Jim Diamond.
+
 2023-08-28  Karl Berry  <karl@freefriends.org>
 
 	* tounicode.c (undump_to_unicode): check that gu->name
diff --git a/texk/web2c/pdftexdir/ptexmac.h b/texk/web2c/pdftexdir/ptexmac.h
index 30e8e744c6..00a32c9688 100644
--- a/texk/web2c/pdftexdir/ptexmac.h
+++ b/texk/web2c/pdftexdir/ptexmac.h
@@ -207,6 +207,10 @@ size_t          T##_limit
     cur_file_name = s;      \
     packfilename(maketexstring(cur_file_name), getnullstr(), getnullstr())
 
+/* This macro doesn't handle the case where a==b. It turns out that is
+ * convenient for the way it's used; for example, see writettf.c.
+ * All uses (must) follow the same pattern.
+ */
 #  define cmp_return(a, b) \
     if ((a) > (b))         \
         return 1;        \