In the middle of the fifteenth century a principal concern of the new sacred genre, the cyclic cantus firmus mass, was the question of musical and liturgical unity. How to balance the quest for unity and the wish for diversty in musical expression or varietas, which Tinctoris advised in his teachings of counterpoint. I take a closer look at an anonymous mass dating from the decade just after 1450, the Missa Sine nomine in MS Cappella Sistina 14, in which the composer was intensely involved with the problem of unity, so involved that he – according to our ideas about music – has focused on ‘unity’ to such a degree that it became rather to the detriment of ‘diversity’. The mass was highly regarded in its time, and this fact puts our aesthetic understanding of the period’s music to test. In addition to the classical analysis of how such a cantus firmus mass is structured as a musical architecture transmitted in writing, we have to ponder how it served as a sounding reality, and how it may have related to the little we know about the musical practices of the period.