Give any tune a second part: a traditional harmony that moves with the tune, or a counterpoint, a busy line of its own that weaves against it. Photograph a tune or paste it in ABC, choose your instrument, and the part is written out under the tune. Then press play: the tune plays, and you play the second part.
You play
Paste a tune on the left and press Write the second part.
playing nowa note you changed
Compare with your original
Click any note in the second part, or the tune note above it, to try other notes there: the list starts with the ones that sound best against the tune and move most easily from the notes either side. Play gives a one-bar count-in, then follows the repeats. Choose whether you play the harmony or the tune, and the other part plays with you.