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 and Dufay.