diff --git a/man7/feature_test_macros.7 b/man7/feature_test_macros.7
index 9f1db40b9..9db8974fa 100644
--- a/man7/feature_test_macros.7
+++ b/man7/feature_test_macros.7
@@ -192,6 +192,46 @@ plus the XSI extension) and C99 definitions.
 definitions for SUSv4 (i.e., the POSIX.1-2008 base specification
 plus the XSI extension).
 .RE
+.IP
+If
+.B __STRICT_ANSI__
+is not defined, or
+.BR _XOPEN_SOURCE
+is defined with a value greater than or equal to 500
+.I and
+neither
+.B _POSIX_SOURCE
+not
+.B _POSIX_C_SOURCE
+is explicitly defined, then
+the following macros are implicitly defined:
+.RS
+.IP \(bu 3
+.B _POSIX_SOURCE
+is defined with the value 1.
+.IP \(bu
+.B _POSIX_C_SOURCE
+is defined, according to the value of
+.BR _XOPEN_SOURCE :
+.RS 7
+.TP
+.BR _XOPEN_SOURCE " < 500"
+.B _POSIX_C_SOURCE
+is defined with the value 2.
+.TP
+.RB "500 <= " _XOPEN_SOURCE " < 600"
+.B _POSIX_C_SOURCE
+is defined with the value 199506L.
+.TP
+.RB "600 <= " _XOPEN_SOURCE " < 700"
+.B _POSIX_C_SOURCE
+is defined with the value 200112L.
+.TP
+.RB "700 <= " _XOPEN_SOURCE " (since glibc 2.10)"
+.B _POSIX_C_SOURCE
+is defined with the value 200809L.
+.RE
+.RE
 .TP
 .B _XOPEN_SOURCE_EXTENDED
 If this macro is defined, and