diff --git a/man2/getgroups.2 b/man2/getgroups.2
index 15a937c3a..1ef73e967 100644
--- a/man2/getgroups.2
+++ b/man2/getgroups.2
@@ -1,4 +1,5 @@
 .\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu)
+.\" and Copyright (C) 2008, 2010, 2015, Michael Kerrisk <mtk.manpages@gmail.com>
 .\"
 .\" %%%LICENSE_START(VERBATIM)
 .\" Permission is granted to make and distribute verbatim copies of this
@@ -187,6 +188,21 @@ supporting 32-bit IDs.
 The glibc
 .BR getgroups ()
 wrapper function transparently deals with the variation across kernel versions.
+.\"
+.SS C library/kernel ABI differences
+At the kernel level, user IDs and group IDs are a per-thread attribute.
+However, POSIX requires that all threads in a process
+share the same credentials.
+The NPTL threading implementation handles the POSIX requirements by
+providing wrapper functions for
+the various system calls that change process UIDs and GIDs.
+These wrapper functions (including the one for
+.BR setgroups ())
+employ a signal-based technique to ensure
+that when one thread changes credentials,
+all of the other threads in the process also change their credentials.
+For details, see
+.BR nptl (7).
 .SH SEE ALSO
 .BR getgid (2),
 .BR setgid (2),