To conform with the titles of the two most famous example, those by Bach, the passacaglia is a continuous variation based on a clearly distinguishable ostinato that normally appears in the bass but that may occasionally be transferred to an upper voice. A chaconne, on the other hand, is a continuous variation in which the "theme" is a scheme of harmonies usually treated so that the first and last chords are fixed whereas the intervening ones can be replaced by substitutes.