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User and system models typically are viewed as independent representations that provide complementary insights into aspects of human-computer interaction. Within system development, it is usual to see the 2 activities as separate, or at best loosely coupled, with either the design artifact or some third "mediatiag" expression providing the context in which the results of modelling can be related. This article proposes that formal system models can be combined directly with a representation of human cognition to yield an integrated view of human-system interaction: a syndetic model. Aspects of systems that affect usability then can be described and understood in terms of the conjoint behavior of user and computer. This article introduces and discusses, in syndetic terms, 2 scenarios with markedly different properties. We show how syndesis can provide a formal foundation for reasoning about interaction.