The design and evaluation of multimodal interaction is difficult. For designers in industry, developing multimodal interaction systems is a big challenge. Although past researches have presented various methodologies, they have addressed only specific cases of multimodality and failed to generalise their methodologies to a range of applications. In this paper, we present a usability framework for the design and evaluation of multimodal interaction. First, in the early phase of multimodality design, elementary multimodal commands are elicited using traditional usability techniques. Second, based on the CARE (Complementarity, Assignment, Redundancy, and Equivalence) properties and the FSM (Finite State Machine) formalism, the original set of elementary commands is automatically expanded to form a more comprehensive set of multimodal commands. Third, this new set of multimodal commands is evaluated in two ways: user-testing and errorrobustness evaluation. This framework acts as a structured and general methodology both for designing and evaluating multimodal interaction. We expect that it will help designers to produce more usable multimodal systems.