Ars nova, as it came to be called, was the invention of a single man, Guillaume de Machaut. Prior to de Machaut, medieval music had been written either in the style of Gregorian chant, as codified by Pope Gregory, or had been written in the style of either of two composers, Leonin or Perotin. Leonin was a French composer who worked at the Cathedral of Notre Dame in Paris, and much of what we know about him comes from an anonymous Englishman. Perotin was a later French composer who worked in the same cathedral, and these two composers codified the system of organum.
De Machaut’s great achievement was to free music from the organum style, allowing greater rhythmic complexity, and to therefore allow for much greater expressiveness in secular music. All the rhythmic styles that had been introduced into sacred music by Leonin and Perotin were now available in secular music, which quickly adopted a form known as the isorhythmic motet.
Pope John the XXIInd denounced the ars nova style, because of the mixture of sacred and secular forms, as well as the inclusion of secular melodies and texts into sacred music, but later it was adopted by Pope Clement IVth. We know of the ars nova style not only from the music of De Machaut, but from the writings in the early fourteenth century of Ars novae musicae [New Technique of Music] (c. 1320) by Johannes de Muris, and Ars nova notandi [A New Technique of Writing [Music] attributed to Philippe de Vitry (c. 1322). This exciting development in the way music was written and notated made a huge stir in the medieval world, and opened the way to what was to come later.