diff --git a/man7/feature_test_macros.7 b/man7/feature_test_macros.7
index d87fea1fa..dde6f0fbb 100644
--- a/man7/feature_test_macros.7
+++ b/man7/feature_test_macros.7
@@ -312,6 +312,13 @@ with a value of 500 or more also produces the same effect as defining
 Use of
 .BR _XOPEN_SOURCE_EXTENDED
 in new source code should be avoided.
+
+Since defining
+.B _XOPEN_SOURCE
+with a value of 500 or more has the same effect as defining
+.BR _XOPEN_SOURCE_EXTENDED ,
+the latter (obsolete) feature test macro is generally not described in the
+SYNOPSIS in man pages.
 .TP
 .BR _ISOC99_SOURCE " (since glibc 2.1.3)"
 Exposes declarations consistent with the ISO C99 standard.