Ockeghem's Missa Prolationum may be the greatest exercise in contrapuntal virtuosity anywhere in the European Renaissance. Strict rhythmic transformation of a notated melody had existed within the tradition of the isorhythmic motet, wherein subsequent sections of the piece would apply such transformations to the tenor's fixed melody by means of new time signatures. Ockeghem's colleague in the 1440s at Our Lady's of Antwerp, Petrus de Domarto, apparently was the first to apply mensural rhythmic transformation to the various movements of a Mass Ordinary cycle, in his Missa Spiritus alme. Guillaume Dufay posed himself the challenge of a "mensuration canon"—two voices singing the same melody in different time signatures—in an early motet, Inclita stella maris, and a late chanson, Les doleurs. But it was left to the supreme craftsman, Ockeghem, to raise the compositional stakes and challenge himself to write an entire mass based on a series of such canons: in short, to exploit the system of mensural notation as never before (and not since). The musical feature which makes a mensuration canon possible is the fact that in the fifteenth century's system of notation, the same note symbol could take a different rhythmic value under different mensurations (or time signatures). This means that in addition to the more standard type of canon, where subsequent voices sing the same melody at a later time or a different pitch level, the same melody may be sung in counterpoint to itself in two different simultaneous time signatures. All of the movements of Ockeghem's Mass are examples of this most difficult type of canon. Moreover, all of the movements are double canons: two notated voices are each subjected to different mensuration canons to produce four vocal parts. As if this weren't enough of a challenge, Ockeghem sets each canon at a different musical interval: Kyrie at the unison, Christe at a whole step, Kyrie II at a third, Gloria at a fourth, etc., to Osanna at the octave. The Agnus Dei movements return to canonic intervals of the fourth and fifth. The Prolation Mass is not merely an intellectual curiosity, however. Music theorists from shortly after Ockeghem's death until the twentieth century have extolled the concept and contrapuntal workmanship of the Mass; the later twentieth century has even seen in Ockeghem a kinship with the hyper-intellectual esotericism of "Contemporary-classical" music written by academic composers. However, the elegance of Ockeghem's Mass is not restricted to the conceptual world of notation and thought; it presents a sonic experience fully commensurate with the harmonies and melodies expected by his contemporary listeners. The openings of movements often seem rhythmically unmoored, as different voices sing long notes of various lengths, but the true test of his skill is the aural facility of his melodic weavings and his cadences; he even successfully slips an expressive accidental into the canonic complex at Crucifixus. A harmonically warmer Sanctus cannot be found in his work, and the octave canon in Osanna provides for a powerful conclusion. Even as natural laws of staggering complexity may govern in secret the processes of a budding rose, a terribly difficult, but hidden, level of craftsmanship yields the sweetness of Ockeghem's music.

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