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We propose the CARE properties as a simple way of characterising and assessing aspects of multimodal interaction: the Complementarity, Assignment, Redundancy, and Equivalence that may occur between the interaction techniques available in a multimodal user interface. We provide a formal definition of these properties and use the notion of compatibility to show how the system CARE properties interact with user CARE-like properties in the design of a system. The discussion is illustrated with MATIS, a Multimodal Air Travel Information System.