This paper describes a high-level extensible language. The language allows the user to define new data types and new operations to suit his needs and to simplify subsequent programming. This work began as an effort to produce a "graphic language", but soon led to the design of a general purpose extensible language in which graphical data types and operations could easily be embedded. A prime aim in the definition of the language was to provide powerful features at a level that is easily understood and yet straightforward to implement. Examples for computer graphics are given to illustrate the most important features of the language. © 1973.