This paper describes and discusses a strategy for the design of an expressive communication aid for the non-vocal which is highly flexible and which embodies the principle of user-specifiable function. Some of the implications which arise when an intuitively common system configuration is adopted in practice are examined. It is shown that a principal feature of the design process and subsequent assessment of the system is the way in which modelling of processes, mechanisms or functions provides a common focus for establishing the important criteria for system design and evaluation of its applicability.