Musica Nova, with its rich experience performing the ballades and motets of Guillaume de Machaut, set out to rewrite the book on his most famous and emblematic work: the so-called Messe de Nostre Dame. The very elaborate construction of the mass and its strange and subtle harmonies win the admiration of all. But do we really know what its true sound was? Though we cannot be quite sure, our research into the 14th-century theory of musica ficta, led by specialist Gérard Geay, has allowed us to come close, opening up an unheard sound world. In order to perform this music the singers worked from various 14th-century manuscript sources. They used the reading techniques of that era in an attempt to stay as close as possible to the phrasing and vocal movement that Machaut would have had in mind. Reading from the manuscript places the singer in a world of long note values with ternary divisions (modus perfectus), which automatically suggests a longer breath and a more spacious performance. Machaut's mass alternates between two styles of composition: the conductus (Gloria and Credo), a 13th-century legacy in which all voices follow the same rhythm, and the isorhythmic motet (Kyrie, Sanctus, Agnus Dei, Ite missa est, and the final Amen in the both the Gloria and Credo), which is specific to the Ars Nova.