Syndetic modelling

User and system models typically are viewed as independent representations that provide complementary insights into aspects of human-computer interact ion. Within system development, it is usual to see the 2 activities as sepa rate, or at best loosely coupled, with either the design artifact or some t hird "mediating" expression providing the context in which the results of m odelling can be related. This article proposes that formal system models ca n 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 properti es. We show how syndesis can provide a formal foundation for reasoning abou t interaction.