Missa prolationum  The Missa prolationum is a musical setting of the Ordinary of the Mass by Johannes Ockeghem, dating from the second half of the 15th century. Based on freely written material probably composed by Ockeghem himself, and consisting entirely of mensuration canons, it has been called "perhaps the most extraordinary contrapuntal achievement of the fifteenth century", and was possibly the first multi-part work written with a unifying canonic principle for all its movements. The mass is for four voices, and is in the usual parts:   A typical performance takes 30 to 35 minutes. Like Palestrina's "Missa Repleatur os meum" (Third Book of Masses, 1570) and the canons of J.S. Bach's "Goldberg Variations" more than two centuries later, the "Missa prolationum" uses progressive canon in all its movements. Most of the movements feature pairs of mensuration canons. The interval separating the two voices in each canon grows successively in each consecutive movement, beginning at the unison, proceeding to the second, then the third, and so forth, reaching the octave at the "Osanna" section in the Sanctus. The four voices each sing in a different mensuration. For instance, in the first "Kyrie", the four voices sing in the meters 2/2, 3/2, 6/4, and 9/4 respectively (in modern notation). Thus, the second voice, in 3/2, sings the same tune as the first voice, in 2/2, but half again as slowly, so the voices gradually pull apart. The same occurs between the second pair of voices, in 6/4 and 9/4 respectively. In the score, only one voice was written out for each canon, with the mensuration marks (approximately equivalent to a modern time signature) given alongside, so the singers would understand that they are to sing in those proportions, and thus at different speeds; in addition, the intervals between the voices are given in the score by the positions of the C clefs. What has so astonished musicians and listeners from Ockeghem's age to the present day is that he was able to accomplish this extraordinarily difficult feat. 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.