X: Why Z?

Window management systems are now used extensively for user interfaces to computer systems. In particular, X11 has come to dominate the workstation market as a widely accepted industry standard on many different hardware platforms. However, no formal standard currently exists for this window system, both in terms of an international standards body (although this is being addressed), and in terms of a precise (mathematical) specification of what the interface is intended to do. This paper advocates the use of a formal notation to describe such an important system to avoid ambiguity and undesired or unintended variations between different implementations of the same system. Theformal notation used for demonstration purposes, Z, is based on set theory, and has been developed at the Programming Research Group in Oxford.