diff --git a/man3/sigsetops.3 b/man3/sigsetops.3
index a515a7b97..32653ded1 100644
--- a/man3/sigsetops.3
+++ b/man3/sigsetops.3
@@ -154,6 +154,14 @@ T}	Thread safety	MT-Safe
 .SH CONFORMING TO
 POSIX.1-2001.
 .SH NOTES
+When creating a filled signal set, the glibc
+.BR sigfillset ()
+function does not include the two real-time signals used internally
+by the NPTL threading implementation.
+See
+.BR nptl (7)
+for details.
+
 .SS Glibc extensions
 If the
 .B _GNU_SOURCE