diff --git a/libs/ardour/session_midi.cc b/libs/ardour/session_midi.cc
index e53970840b..6d7fea3b26 100644
--- a/libs/ardour/session_midi.cc
+++ b/libs/ardour/session_midi.cc
@@ -340,6 +340,9 @@ Session::get_midi_nth_route_by_id (PresentationInfo::order_t n) const
 {
 	PresentationInfo::Flag f;
 
+	/* These numbers are defined by the MMC specification.
+	 */
+
 	if (n == 318) {
 		f = PresentationInfo::MasterOut;
 	} else if (n == 319) {