Masses by Ockeghem—works with which his historical reputation, for better or worse, is indissolubly
bound up.
One of them is called the Missa Prolationum, the “Mass of the Time Signatures.” It is sung in four
parts but written in two, both to be simultaneously realized as canons in an ascending cycle of intervals:
the first section of the Kyrie is a double canon at the unison; the Christe at the second; the second Kyrie at
the third; the Gloria at the fourth; the Credo at the fifth, and so on. The peculiar title advertises the fact that
each of the two voices as written carries a double time signature: and when the Mass is actually sung,
each of the four voices realizes its note-values according to a different mensuration scheme.
By writing canons in which the voices are in effect singing at different speeds, Ockeghem is able to
start with all the voices singing together. When sufficient distance has been achieved between the canonic
voices, Ockeghem employs additional notational devices that effectively neutralize the differing time
signatures, and the canons proceed as normal ones. Only one little item in the Missa Prolationum, a duo,
is truly a “mensuration canon,” in which two parts derived from a single notated line move at different
rates of speed throughout. In Ex. 12-7 that duo, the second Agnus Dei, is given in its original notation and
in a two-voice realization. The lower voice reproduces the upper voice an octave below and at half the
speed, and consequently ends halfway through the written part. This strict little duo, one of the simpler-
textured items in the Mass, will give an idea both of its diabolically clever contrivance, and also of the
smooth mellifluousness of the result (the truest art, to recall Horace’s famous dictum once again, being the
art of concealing art).
EX. 12-7A Missa Prolationum, Agnus II, in the original notation as a single voiceEX. 12-7B Missa Prolationum, Agnus II, realized as a mensuration canon in two parts