The volume publishes for the first time the text of the two Italian translations of the Ars practica mensurabilis cantus secundum Iohannem de Muris, both preserved in a single manuscript and written in Florence, the oldest one at the end of the XIV century, the other about a century later, along with an extensive introductory study. In the appendix, is provided the text of the Latin version of the treatise from which the second translation is derived (that is excluded from previous editions) and a musical glossary.