Chapel master Franchinus Gaffurius supervised the making of four voluminous choirbooks for the musical chapel at Milan Cathedral, the epicentre of the Ambrosian Rite. However, many pieces within the Gaffurius codices correspond to the Roman Rite or blend characteristics of both rites. This study examines the unexplained ritual dualism in the Mass ordinary settings from various liturgical perspectives, including institutional and ritual history, as well as codicological, textual and musical analyses. It suggests that this polyphony served as an additional layer to the liturgy to promote devotion and offers pragmatic explanations for the apparent ritual contradictions. Additionally, this study analyses the relationship between music and text in the rarely studied Mass ordinary settings of Gaffurius, some of which are autographs. It reveals his compositions as musical exegeses of their text through contrapuntal techniques, text omissions, and structural divisions. Although the specific Milanese context requires tailored methods, the results contribute to our general understanding of polyphonic Mass settings as a medium of text and devotion in the Renaissance.