Ars antiqua, as it later came to be known, was the style of music that included both Gregorian chant, as codified by Pope Gregory, and the later, more rhythmical styles of composition as published by two great French composers, Leonin and Perotin the Great. These two composers developed a style of composition that added more voices and rhythmical complexity to the older Gregorian chant style, and paved the way for future developments in music.
Leonin was the founder of the Notre Dame School of polyphonic composition, and was later succeeded by Perotin the Great at the cathedral. Both composers transformed the Gregorian chants into motets, in which each voice had a different melody and text; Perotin is the first composer whose work still survives, who wrote for four voices.
Ars antiqua covers the period of music between approximately 1170 A.D. and 1310 A.D., and saw the first formal rhythmic notation, and the rules of the rhythmic modes were drawn up during this period. In addition, the ars antiqua era of music was the foundation of the beginnings of modern music notation, in which differently shaped notes denoted different durations and rhythms.
The ars antiqua movement of medieval came to a screeching halt with the introduction of the ars nova style. The main composer of ars nova style music was Guillaume de Machaut. Ars antiqua (old art) was the term given to this style of composition after Guillaume de Machaut, when music had already evolved into the ars nova (new art) style. Of course the composers of ars antiqua music considered their own style new and exciting!