This paper considers contratenor parts in equal-cantus Italian motets found in the manuscript Bologna Q.15 from codicological, contrapuntal and stylistic perspectives. After establishing criteria for contratenor classification, the paper collates a lexicon of rules governing contratenor behaviour in fourteen motets by Johannes Carmen, Johannes Ciconia, Christoforus de Monte, Antonius Romanus, Matheus de Brixia, and Antonius de Civitate. Four paradigmatic motets are presented as case studies. Q.15 is the earliest (or only extant) source for many of these contratenors, several of which were probably added by a later composer, and they exhibit remarkably similar behaviour that we might term a ‘Q.15 style’. Finally, the lexicon is employed to distinguish between the features of an original contratenor and a later one, and the paper posits that studying contratenors can offer a unique window into fifteenth-century compositional thinking.