We have in Period IIC traced the work of the musicians of the medieval Church from the majestic monody of Gregorian chant through ever great elaborations to a fully fledged form of secular polyphony. But this too had its foundations in the chant of the liturgy. By the latter part of the 13th century however the plainsong tenor,once the dominant element in the music, was becoming nothing more than a device of composition. Already, in a great many motets in the Montpellier and Bamberg manuscripts, the syllables of the text of this tenorare no longer placed under the notes. The scribe contents himself merely with mentioning the opening word, apparently as a reference. From this it was natural to dispense with the words altogether and give the tenor to an instrument. Further developments robbed the plainsong tenor not only of its words but also of its formal integrity. In the isorhythmic motet it became fragmented into sections of identical rhythm. In yet other forms the tenorwas reduced to the briefest of phrases, now barely identifiable, which were repeated several times over during the composition.
French music throughout history has been preoccupied with order, and in the 14th century the most notable manifestation of that concern was the principle of isorhythm. Its origins date back to the century before, when Pérotin and his followers had noticed that to achieve the desired length in their substitute clausulae,the chant tenor had to be repeated. By organizing the notes into rhythmic patterns; they gave the tenor line shape, and increased the audibility of the chant. Musicians saw that two separate entities were at work: the rhythmic pattern, termed the talea("cutting"), and the series of melodic intervals, termed the color("repetition"). If sections of colorand taleaof different lengths are chosen, the patterns automatically overlap; each time they repeat, they coincide at different places. Moreover the notes of the taleamight be augmented to double their length or diminished to halve it, to mention but two of the simpler possibilities. Nor did composers confine isorhythm to the tenor. As time went on, isorhythmic techniques began pervading the upper voices; typically, these voices would have their own rhythmic and/or melodic patterns.
The isorhythmic motet became the major art form of the French Ars Nova. Its appeal is easy to fathom. Isorhythm enabled the tight organization of extended works, no small advantage in pieces which can baffle all but the most informed listener with their surface complexity of polyrhythms, polymeters, and polytextuality. Isorhythm also satisfied the medieval penchant for abstraction and concealed meaning. Only the initiated could consciously discern what the composer had done, though even a casual listener could subconsciously sense some sort of underlying coherence.