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.