Late medieval music saw a backlash against the complexity of the motet, such as this statement made by Pope John XXII, issued from Avignon 1324-25, attacking current practices in musical composition:
There are certain disciples of the new school who, devoting all their attention to measuring time, apply themselves to the making of notes in a different fashion. They prefer to compose their own songs rather than to sing the old ones, and divide the church pieces into semibreves and minims; they chop up the chant with notes of short values, truncate the melodies with hockets, pollute the melodies with descants and go as far as to muffle the upper voice in the vulgar tongue. Thus they ignore the principles of the antiphonal and the gradual, ignore the tones, which they no longer distinguish and even mingle together; under this avalanche of notes the chaste ascensions and discreet closes of the plainsong, by which the tones themselves are distinguished become unrecognizable.The Pope was condemning some of the new 14th-century practices, what theorist and composer Philippe de Vitry (1291-1361) coined as "Ars Nova" -- the new art. Currently, this movement refers to France (with Italian innovations now considered its own phenomenon: the Trecento).
De Vitry also outlines 14th-century mensuration (the precursor to time signatures). The tempus is the grouping of beats, designated by a perfect circle or by an imperfect incomplete circle; the prolatio the division of beats designated by a dot inside the circle or its absence. So a circle with a dot = triple grouping, triple division, like 9/8 time signature. A circle without a dot means triple grouping, double division, like 3/4 time. An incomplete circle with a dot means double grouping, triple division, like 6/8 time. And an incomplete circle without a dot means double grouping, double division, like 2/4 or 4/4 time.
Most distinctive in the 14th century was the use of "isorhythm" as a technique of composition, appearing in masses, motets, and "fixed forms." Isorhythms started in the tenor line but spread complexly to other parts. An isorhythmic piece uses repeated rhythmic patterns that often involve repeated melodic patterns too. The "talea" refers to the rhythm repeated; the "color" is the repeated pitch pattern. (Occasionally these may coincide, but then that's simply an entire repeat section.)
The "formes fixes" ("fixed forms") include the following:
The Italian version of Ars Nova is called Trecento (short for Mille Trecento = 1300s), and it too has secular fixed forms:
Later, in the late 14th and early 15th centuries, Ars Nova and Trecento composition became somewhat "mannerist" and is called "Ars Subtilior" (Subtler Art). (After any art movement is fully explored it eventually just seems gaudy. Mannerists in the visual arts became concerned with arabesques, decorative designs, wild colors and symbols, produced for the dying nobility going to extremes in dress and manner of life.)
In the music, interest in puzzles and pictograms led to eccentric and visually stimulating notation and even more complex rhythms. Baude Cordier's "Tout par compas" is composed in circles. "Belle bonne, sage" is a rondeau presented in a heart-shaped score in the Chantilly manuscript.
This puts us on the verge of the Renaissance and Renaissance music now.