The Syzygy software library consists of tools for programming VR applications on PC clusters. Since the PC cluster environment presents application development constraints, it is impossible to simultaneously optimize for efficiency, flexibility, and portability between the single-computer and cluster cases. Consequently Syzygy includes two application frameworks: a distributed scene graph framework for rendering a single application?s graphics database on multiple rendering clients, and a master/slave framework for applications with multiple synchronized instances. Syzygy includes a simple distributed OS and supports networked input devices, sound renderers, and graphics renderers, all built on a robust networking layer.