This chapter has two main purposes. It provides a selective review of accidentals in polyphonic chansons by Guillaume de Machaut, and it covers a few basic aspects of the polyphonic chanson, such as will be relevant for the remainder of our survey. To leap directly from the grand chant courtois to Machaut's polyphonic formes fixes requires that we gloss over what was probably a complicated transition; it is impossible to say exactly how complicated, since the transition is documented so poorly in the scant surviving sources. The polyphonic chanson may well have developed along several independent paths. There is an unusual anticipation of the idiom in Trouvère MS O, for example: a trouvère song similar to others in the manuscript is accompanied by a part labeled “tenor” and arranged like a motet tenor, with a regular rhythmic pattern. The example suggests one way of thinking about the polyphonic chanson: the idiom represents a synthesis, of sorts, between a tradition of lyric melody, inherited from the trouvères, and the increasingly complex polyphony of the motet. In his mature style, Machaut does indeed arrive at a successful synthesis of lyric melody with counterpoint. The idea of the polyphonic chanson as a synthetic creation suggests a way to think about accidentals. As a rule, the accidentals that we shall encounter in polyphonic songs have something to do with basic and universally recognized formulas of counterpoint. Yet the formulas are used in ways that call to mind the model of discursive melody sketched out in our study of trouvère song. Selective use of the contrapuntal formulas is what the sources indicate, often enough.