diff --git a/doc/mkvmerge.1 b/doc/mkvmerge.1
index 200d6dda3..13bc42141 100644
--- a/doc/mkvmerge.1
+++ b/doc/mkvmerge.1
@@ -334,7 +334,9 @@ normally calculates. Read the section about \fBEXTERNAL TIMECODE FILES\fR.
 This option controls to which track another track is appended.
 Each spec contains four IDs: a file ID, a track ID, another file ID and
 a second track ID.
-The first pair identifies the track that is to be appended, the second pair
+The first pair, "source file ID" and "source track ID", identifies the track
+that is to be appended.
+The second pair, "destination file ID" and "destination track ID", identifies
 the track the first one is appended to.
 .br
 If this option has been omitted then a standard mapping is used.