diff --git a/man7/feature_test_macros.7 b/man7/feature_test_macros.7
index d8c6134c9..527bf733c 100644
--- a/man7/feature_test_macros.7
+++ b/man7/feature_test_macros.7
@@ -139,6 +139,12 @@ The macros that you most likely need to use in modern source code are
 .BR _DEFAULT_SOURCE
 (to get definitions that would normally be provided by default).
 .IP *
+Certain macros are defined with default values.
+Thus, although one or more macros may be indicated as being 
+required in the SYNOPSIS of a man page,
+it may not be necessary to define them explicitly.
+Full details of the defaults are given later in this man page.
+.IP *
 Defining
 .BR _XOPEN_SOURCE
 with a value of 600 or greater produces the same effects as defining