The Tractatus de tonis of Guy of Saint-Denis (written ca. 1300-10)differs from other treatises on plainchant in the depth of its analysis of thevarious tones into which chant was traditionally classified. The treatise ofGuy of Saint-Denis is preserved as the concluding item in an anthology of textsabout plainchant that Guy compiled (now London, British Library, MS Harley281), beginning with some writings of Guido of Arezzo and a Cistercian tonary,but also including the Ars musice ofJohannes de Grocheio and the Tractatus detonis of Petrus de Cruce. Guy's treatise presents itself as a syntheticoverview of both the theory and practice of plainchant in a way that combinesthe very practical reflection of Guido of Arezzo with ideas of more Aristoteleianinspired theorists such as Johannes de Grocheio and Peter of Auvergne.
