In the years after Schoenberg's death, many musicians claimed that in consistency of method, in systematization of the total fabric, and in originality, Schoenberg must defer to Webern. Particularly in Europe, composers took Webern as their model and extended ideas found in his twelve-tone works. This cultivation of Webern's twelve-tone technique justified itself in part by portraying Schoenberg as still enmeshed in nineteenth-century shackles.' We are accustomed, for instance, to speak of a post-Webern school but not of a post-Schoenberg school. The body of important music and theoretical writing responding specifically to Schoenberg's challenge has gradually corrected this view of Schoenberg's contribution. The notion of combinatoriality, with its far-reaching consequences in harmonic, developmental and formal spheres, derives directly from Schoenberg's compositional methods and is only one example. Despite his now widely acknowledged contribution, Schoenberg's rhythm is often still disdained as exemplifying his inability to free himself from outmoded methods. A careful examination of his works, however, in particular the Fourth String Quartet, will reveal systematic use of rhythmic and partitioning techniques to create invariants. Procedures of this sort, which we shall term isomorphic partitioning, are used so extensively as to constitute a major feature of the