Evidence is offered to support the view that linguistic competence cannot in principle be divorced from linguistic performance in order to abstract universal properties of grammars, that rules of grammar inevitably incorporate perceptual strategies and constraints, and that grammaticality and acceptability are related to predictability. A theory of systematic variation affords direction for gathering data on rule-governed language use and for representing the results in formal grammars that predict speech behavior. Some of the strategies and constraints operating in performance and the rule-governed regularities they produce are demonstrated in the analysis of seven tape-recorded task-oriented dialogs. (Author)