Continuities within and between historical periods are often contentious, and this new book offers a stimulating perspective on the point of demarcation between ‘Medieval’ and ‘Renaissance’. At issue is the potential, made possible by mensural notation, for the same written material to yield different musical results in performance. The discrepancy between written material and sonic results only appears that way to us because of music notation’s subsequent development, as the relative duration of musical signs became fixed; to those working within the mensural system, the notation of duration was always understood to be contextually dependent. But over the course of the fifteenth century composers began to engage collectively with that potential with ever increasing intensity, ingenuity, and sophistication. Where Sight Meets Sound focuses on an area where this phenomenon found particularly rich expression: the notation and performance of cantus firmi. One might think that this potential had always existed, being written into the mensural system itself. The first surprise of Emily Zazulia’s narrative is the realization that its exploitation as a compositional resource came about only gradually. She begins with a thought experiment: Charles the Bold, duke of Burgundy, having inherited the now fragmentary Trémoïlle manuscript first owned by his great-grandfather, asks his court composer Antoine Busnoys to re-notate some of the music so that his chapel can sing it. Confronted with tenors incorporating repetition, Busnoys and his singers would have found the notation alien not because it was too complex but because it was in a certain sense not complex enough. Although it was quite common in fourteenth-century tenors for material to be repeated in halved note values (diminution, in other words), Zazulia shows that in such cases scribes generally wrote out the diminutions out in full, whereas by Busnoys’s time they were typically indicated by means of a repeat sign, a verbal rubric, or some other ‘metasign’ (Zazulia’s term—not just for reasons of practicality or economy (e.g. space-saving) but as a positive aesthetic decision. In the first chapter, Zazulia locates the change in scribal practice in the first quarter of the fifteenth century; not coincidentally, the same period sees the emergence of pieces (like the much-discussed Codex Chantilly motet Inter densas/Imbribus/Admirabile) where repetition no longer consists of ‘straight’ or exact diminution (in modern terms) but entails a degree of contextual reinterpretation, brought about by reading the same material under different mensurations. (Another harbinger of the change in approach is scribes’ increasing tendency to notate mensural signs at the start of pieces.) The potential for prescribing non-standard readings of the notation (Zazulia’s ‘transformative impulse’) gradually extended to other systematic procedures (e.g. augmentation, inversion, retrograde) and thence to more arbitrary operations in the last quarter of the fifteenth century (e.g. discounting of stems, certain categories of pitches, or rests). But the ‘transformative impulse’ is often accompanied by a corollary, the principle of ‘notational fixity’ (Zazulia’s term again), whereby all these transformations refer back to a single notational artefact whose details were fixed for the duration of the work. As the genre most associated with multiple repetitions of borrowed material, the cantus firmus mass became a privileged locus of these strategies. It has been observed that notational fixity was first associated with mass tenors derived from polyphonic songs (understandably, since these dictated both pitches and rhythms); only later, particularly with Obrecht, was the principle extended to chant-based tenors (particularly with respect to the ligatures of the given source), whose use as cantus firmi of course pre-dated that of polyphonic songs.