Dialogue independence -- the decoupling of the Computer-Human Interface from the core application software -- can be achieved simply through an appropriate architectural framework, with no loss of efficiency. We show that the objective of dialogue independence can be decomposed into three separate subgoals that a software architecture must resolve: existence, property, and transition. We identify architectural patterns that satisfy all three subgoals, and give a rough sketch of their design and implementation. We chose an air-traffic-control system to illustrate our proposed decomposition because of our experience with it and because it exposes many of the difficulties inherent in a typical, large CHI software architecture. We use the terminology of object-oriented software architecture, but we propose a decomposition that is independent of the methodology used.