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Interactive systems involve both events, which occur at specific moments (e.g. keystrokes, mouse clicks and beeps), and more persistent "status phenomena", which can be observed at any time (e.g. the position of the mouse or the image on the screen). Most formalisms used for interactive systems concentrate on one aspect or the other, and may be asymmetric in their treatment of input and output. Notations and models for interface specification are classified in this paper by the way they treat status and event phenomena in their input and output. This is used to construct a model and an associated notation which incorporates both. By specifying examples using this model, important design issues are highlighted which would be missed if either the status or the event phenomena were not properly treated.