+.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).