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This article introduces and discusses a specific grammatical structure—the Command Language Grammar (CLG)—as a representational framework for describing the user interface aspects of interactive computer systems. CLG partitions a system into a Conceptual Component (tasks and abstract concepts), a Communication Component (command language), and a Physical Component (display, keyboard, etc.), The components are further stratified into distinct Levels—a Task Level, a Semantic Level, a Syntactic Level, and an Interaction Level-each Level being a complete description of the system at its level of abstraction. Each Level's description contains procedures for accomplishing the tasks addressed by the system in terms of the actions available at that Level. That is, the system is described by progressive refinement. An extensive example, a small message-processing system, is described at all Levels in …