diff --git a/man7/pthreads.7 b/man7/pthreads.7
index 98eb86187..906b5a08c 100644
--- a/man7/pthreads.7
+++ b/man7/pthreads.7
@@ -127,8 +127,10 @@ Thread IDs are guaranteed to be unique only within a process.
 that ID by definition refers to a thread in
 the same process as the caller.)
 
-A thread ID may be reused after a terminated thread has been joined,
+The system may reuse a thread ID after a terminated thread has been joined,
 or a detached thread has terminated.
+POSIX says: "If an application attempts to use a thread ID whose
+lifetime has ended, the behavior is undefined."
 .SS Thread-safe functions
 A thread-safe function is one that can be safely
 (i.e., it will deliver the same results regardless of whether it is)