The organ sections of alternatim masses in the fifteenth century are not arrangements of pre-existent polyphonic works but instead involve a newly composed duplum of an instrumental texture set above the Gregorian chant tenor. The organ alternates with the chorus that sings the chant in unison rather than with a polyphonic composition. This process of composition reveals an astonishing originality because the model for organ compositions comes from the organum of the twelfth century, a historical distance of three hundred years. Although it may seem strange that vocal organum could inspire fifteenth century organ music, there is evidence that suggests this vocal idiom was in use over a longer period of time in a number of European regions. There were also phases in the development of the organ mass, the first of which involved an elaborate duplum against the unrhythmical and sustained tenor. In the second stage, the tenor became more rhythmicized as a way of coordinating harmonies but usually only during limited sections of clausulae. The third development is the conductus style in which both voices move in chords, a form that is idiomatic to the instrument.

Index Classifications: 1400s

Contributed by: Mary Ellen Ryan