The presumption of a notated musical “score” as the subject for realization by the performer – and the object of analysis by the theorist – has become a foundation of Western musical aesthetics, one whose ontology underlies much of the theory described in the present volume. It is clear, however, that a great deal of music has been based not upon written scores, but rather upon oral transmission and traditions of improvisation. This is most evident, of course, in popular and non-Western repertories. But it is also true of much Western “art” music, particularly during medieval times, when a precise notation had yet to develop. Even after such a notation gradually did evolve, though, a large degree of improvisational freedom continued to be practiced in many different repertories and styles. The result is that the distinction between composer and performer in such music is blurred, if not non-existent. In essence, the musical “work” is the performance.

Improvisational performances are rarely arbitrary. Most genres of music having extempore elements commonly presume guidelines of syntax and style that constrain performers. These guidelines – sometimes explicitly formulated, sometimes informally so – become “theories” that can be understood as historical counterparts to the more formalized prescriptive rules that guide the composition of written scores. Still, the distinction is not always a clear one, and many treatises, particularly in the early modern period, blur the guidelines between written, improvised, and “realized” musics. The resultant theories are often as complex and intricate as are the musical structures for which they purport to account.