It is a term to denote a structural principle frequently used
in 14th-century motets, particularly in the tenors. Its main feature
is the use of a reiterated scheme of time values for the presentation of
a liturgical cantus firmus. The isorhythmic principle, although usually
thought of as a characteristic feature of the ars
nova, is the logical development of the modal
rhythm of the 13th century. Modal patterns differ from the 14th-century
"taleae" (rhythm patterns) only in length.
Particularly interesting are examples in which the number of notes in "color"
(the melodic pattern) and "talea" are not in proportion, thus leading
to the repetition of the melody in different rhythmic patterns. As
an example, if the "color" includes nine notes and the "talea" five, the
"color" would have to be repeated five times until both schemes would come
to a simultaneous close. Many pieces, however, come to a close, leaving
the last repetition of the "talea" unfinished.
Such configurations, involving overlapping of the "taleae"
and the "colores," are also found in some 14th-century motets.
In the 14th century the isorhythmic principle was not only the chief method
for the rhythmic organization of the tenors but was also applied, more
freely, to the upper parts. In most of Machaut's
motets the upper parts have identical rests at corresponding places of
the "talea", so that the periods within one "talea" are of the same length
as those within the others (isoperiodicity). The culmination of this
development is the panisorhythmic motet, in which every voice-part is strictly
isorhythmic. A panisorhythmic motet may be said to consist of a number
of "melody-variations" of a fixed rhythmic theme. This type is the
one normally used by the later composers of isorhythmic motets, Dunstable