The treatise on musica plana and musica mensurabilis written by Lambertus/Aristoteles is our main witness to thirteenth-century musical thought in the decades between the treatises of Johannes de Garlandia and Franco of Cologne. Most treatises on music of this century - except for Franco's treatise on musical notation - survive in only a single copy; Lambertus's Ars musica, extant in five sources, is thus distinguished by a more substantial and long-lasting manuscript tradition. Unique in its ambitions, this treatise presents both the rudiments of the practice of liturgical chant and the...