diff --git a/include/osmocom/msc/sdp_msg.h b/include/osmocom/msc/sdp_msg.h
index 98d1892..cf1e560 100644
--- a/include/osmocom/msc/sdp_msg.h
+++ b/include/osmocom/msc/sdp_msg.h
@@ -10,6 +10,7 @@
 int sdp_subtype_name_to_payload_type(const char *subtype_name);
 
 struct sdp_audio_codec {
+	/* Payload type number, like 3 for GSM-FR. */
 	unsigned int payload_type;
 	/* Like "GSM", "AMR", "EFR", ... */
 	char subtype_name[16];