The sixth may be major (9 semitones) or minor (8 semitones). Its sound is soft, warm, and song-like; composers therefore often begin melodies with an upward leap of a sixth (the “from afar” quality of many lyrical themes). The major sixth sounds bright and joyful, the minor tender and a little wistful.
In harmony, the six-three chord (a triad inversion) lends smoothness and avoids a heavy bass. Parallel sixths are a favourite device of duet singing — including the Ukrainian folk tradition, where two voices move together to create the same “third-and-sixth” fullness by which Ukrainian song is recognized the world over. The sixth is the interval of the heart.
