Software architects use a number of commonly-recognized "styles" to guide their design of system structures. Each of these is appropriate for some classes of problems, but none is suitable for all problems. How then, does a sofmare designer choose an architecture suitable for the problem at hand? Two kinds of information are required: (1) careful discrimination among the candidate architectures and (2) design guidance on how to make appropriate choices. Here we support careful discrimination with a preliminary classification of styles. We use a two-dimensional classification strategy with control and data issues as the dominant organizing axes. We position the major styles within this space and use finer-grained discriminations to elaborate variations on the styles. This provides a frame-work for organizing design guidance, which we partially flesh out with rules of thumb.