Burgmüller Op. 100, no. 7 The Clear Stream
In the first section, the thumb plays the melody, which is immediately echoed by the middle note of the triplet, usually a sixth above. Note that the melody note is both part of the triplet and a quarter note, so it should be held throughout the count. The left hand takes the melody in the second section, with the right hand echoing at a distance of a third. Notice that every triplet in the first section contains a D (except one), and, more obviously, each triplet in the second section has an A.