diff --git a/usr.bin/comm/comm.1 b/usr.bin/comm/comm.1
index c054428eaee2..fb1a11e7470c 100644
--- a/usr.bin/comm/comm.1
+++ b/usr.bin/comm/comm.1
@@ -107,6 +107,12 @@ The
 .Nm
 utility conforms to
 .St -p1003.2-92 .
+.Pp
+The
+.Fl i
+option is an extension to the
+.Tn POSIX
+standard.
 .Sh HISTORY
 A
 .Nm