Almost all composers of the ars antiqua are anonymous. Léonin (fl. late 12th century) and Pérotin (fl. c.1180 – c.1220) were the two composers known by name from the Notre Dame school; in the subsequent period, Petrus de Cruce, a composer of motets, is one of the few whose name has been preserved.
In music theory the ars antiqua period saw several advances over previous practice, most of them in conception and notation of rhythm. The most famous music theorist of the first half of the 13th century, Johannes de Garlandia, was the author of the De mensurabili musica (about 1240), the treatise which defined and most completely elucidated the rhythmic modes. A German theorist of a slightly later period, Franco of Cologne, was the first to describe a system of notation in which differently shaped notes have entirely different rhythmic values (in the Ars Cantus Mensurabilis of approximately 1260), an innovation which had a massive impact on the subsequent history of European music. Most of the surviving notated music of the 13th century uses the rhythmic modes as defined by Garlandia.
The ars antiqua is sometimes divided into two rough periods, known as the early Gothic and the high Gothic. The early Gothic includes the French music composed in the Notre Dame school up until about 1260, and the high Gothic all the music between then and about 1310 or 1320, the conventional beginning of the ars nova. The forms of organum and conductus reached their peak development in the early Gothic, and began to decline in the high Gothic, being replaced by the motet.
Though the style of the ars antiqua went out of fashion rather suddenly in the first two decades of the fourteenth century, it had a late defender in Jacques of Liège (alternatively Jacob of Liège), who wrote a violent attack on the "irreverent and corrupt" ars nova in his Speculum Musicae (c.1320), vigorously defending the old style in a manner suggestive of any number of music critics from the Middle Ages to the present day. To Jacques, the ars antiqua was the musica modesta, and the ars nova was a musica lasciva—a kind of music which he considered to be indulgent, capricious, immodest, and sensual (Anderson and Roesner, 2001).