This paper gives an informal presentation of the language currently used at the Programming Research Group. Oxford University for documenting the specification and the design of computer systems. The language is an extension of conventional set theoretical notation. The extensions are simple syntactical conventions and shorthands, which are introduced in order to improve the writebility and readability of specifications. The main aim in the development of this new language has been to provide a uniform notation and a formal framework for reasoning and proving properties about computer systems. The language (in its current form) has been shown to meet this aim (e.g. [Sufrin. 1]).