Ars nova

from Italian manuscript J. IV.115,
an example of Ars Nova notation

Beginning in the early 1300s there was a change in musical style, an evolution from monophony (a single melody) to polyphony, in which two or more lines of melody intermingled. The result was to give music a richer, more expressive sound.

The Church didn't like it.

Pope John XXII rejected it (as he tried to reject elections of which he did not approve). The sacred monophonic chant of the Church was being mixed with secular tunes. Music was becoming "fancy" and "frivolous" in ways that did not suit the pope.

The new style caught on, however, and there was no turning back. Two books describing the new technique helped to spread the new ideas. They were Ars novæ musicæ [New technique of music] by Jean de Muris c. 1320, and Ars nova notandi [New technique of musical notation] by Philippe de Vitry in about 1322. Because of these titles, 20th century historians refer to this style and period of time in music (the 14th century) as the Ars Nova. This new style developed at the same time in France and Italy. In France, one of its greatest exemplars was the poet Guillaume de Machaut. A sample of his musical composition is found in this post.

Among the new forms of non-sacred music given to us by the Ars Nova are the Madrigal, usually a song of love for two voices, and the Ballad, a story with a non-religious theme which was meant to be sung in public. The music in the manuscript of the Roman de Fauvel is an example of Ars nova.