The problem of modally ambiguous chants – which received much attention from medieval theorists – is most clearly demonstrated by juxtaposing different readings of a particular chant. This procedure often reveals transpositions (partial or complete), and other differences, many of which affect the final cadence. For example, the offertory Justitiae Domini, in the Vatican edition and various manuscript sources (e.g. F-Pn lat.776, 780, 1132) ends on E (deuterus); but the St.Yrieix gradual (F-Pn lat.903) and some other early sources (e.g. I-BV VI.34, GB-Lbl Harl.4951) emend the final cadence to end on F (tritus). This changes the maneria of the chant. A critical study of such mode-change is the primary task of this paper.