Ockeghem was the first composer of canons at the second, third, sixth, and seventh (the "imperfect" intervals), and the "Missa prolationum" may have been the first work to employ them. Its format, with the interval of imitation expanding from the unison up to the octave, was used by Bach in the "Goldberg Variations", but it is not known whether Bach knew Ockeghem's work (which was generally unavailable in the 18th century). Another unusual feature of this mass is that the melodies used for its canons were all apparently freely composed; none have been identified as from other sources. In Ockeghem's time, composers usually built masses on preexisting tunes such as Gregorian chant or even popular songs.