While it is no longer unusual to notice that Schoenberg's work has profoundly affected our musical frame of mind, it is always worthwhile to consider how it has done so. As his challenge to traditional musical assumptions has encouraged depletion of our stock of normative answers to normative questions, it has consequently increased activity in the interface between composition and analysis. Now more than ever, analytic ideas are influenced by our compositional experiences, and compositional notions are inspired by analytic observations, as rules within, rather than about, pieces seem to resupply our inventory. Though this is a healthy state of musical being there have been certain problems in this exchange. The complexion of a musical concept is very different when that concept is viewed as a compositional apriority, than when considered as a means of describing musical sense. By its nature, the former notion must be subjected to interpretation and transformation to claim its compositional value, while a main purpose of the latter is specifically to explicate this process. The identification and definition of a musical concept known as a pitch class, for example, descended neatly from the world of the twelvetone system where it is axiomatic in the definition and construction of twelve-tone relations. Though the idea of pitch class, more generally octave equivalence, certainly predates Schoenberg, its post-Schoenbergian definition is significant, especially in that it facilitated the subsequent definition of musical concepts which do not have such highly formed precedents, concepts such as "collection," "normal form," "aggregate," "inversion," "intersection," "interval class," etc., all of which have become central to our compositional as well as our analytical vocabularies. The compositional use of such concepts places them in a relatively uninterpreted position with respect to a composition since the composer must make pitch, rhythmic, and timbral assignments, or "interpretations," each of which constitutes only one of many possible choices within each cate-