diff --git a/man2/getpid.2 b/man2/getpid.2
index e6c4d96fa..e894302fa 100644
--- a/man2/getpid.2
+++ b/man2/getpid.2
@@ -59,6 +59,18 @@ If the caller's parent is in a different PID namespace (see
 .BR pid_namespaces (7)),
 .BR getppid ()
 returns 0.
+.PP
+From a kernel perspective,
+the PID (which is shared by all of the threads in a multithreaded process)
+is sometimes also known as the thread group ID (TGID).
+This contrasts with the kernel thread ID (TID),
+which is unique for each thread.
+For further details, see
+.BR gettid (2)
+and the discussion of the
+.BR CLONE_THREAD
+flag in
+.BR clone (2).
 .\"
 .SS C library/kernel differences
 From glibc version 2.3.4 up to and including version 2.24,