diff --git a/man7/feature_test_macros.7 b/man7/feature_test_macros.7
index e6b9fba47..82502fb98 100644
--- a/man7/feature_test_macros.7
+++ b/man7/feature_test_macros.7
@@ -529,16 +529,37 @@ and
 .BR _SVID_SOURCE .
 .TP
 .B _REENTRANT
-Historically,
-defining this macro exposed definitions of certain reentrant functions.
-However, already for many years this macro has had few effects,
-and since glibc 2.3, no effects that were not also achieved by defining
-.BR _POSIX_C_SOURCE
-with a value of 199606L or greater.
-This macro is now deprecated; since glibc 2.25,
-defining it has the same effect as defining
+Historically, it was necessary to define this macro in all
+multithreaded code.
+(Some C libraries may still require this.)
+In glibc,
+this macro also exposed definitions of certain reentrant functions.
+
+However, glibc has been thread-safe by default for many years;
+since glibc 2.3, the only effect of defining
+.BR _REENTRANT
+has been to enable one or two of the same declarations that
+are also enabled by defining
 .BR _POSIX_C_SOURCE
 with a value of 199606L or greater.
+
+In glibc 2.25 and later,
+.B _REENTRANT
+is deprecated.
+Defining it is equivalent to defining
+.B _POSIX_C_SOURCE
+with the value 199606L.
+If a higher POSIX conformance level is
+selected by any other means (such as
+.B _POSIX_C_SOURCE
+itself,
+.BR _XOPEN_SOURCE ,
+.BR _DEFAULT_SOURCE ,
+or
+.BR _GNU_SOURCE ),
+then
+.B _REENTRANT
+has no effect.
 .TP
 .B _THREAD_SAFE
 Synonym for the (deprecated)