AbstractThe C language is a fine tool for writing compact and efficient programs. It is relatively easy to produce good compilers for, and the number of tools available for supporting program‐ ming in C is large, especially in its ‘home environment’, the UNM system. However, C'S facilities for structuring programs were, until recently, rather limited. To remedy this situation, a data abstraction facility, called classes, was added. The class concept described here has benefitted from the experience gained through a year's use. It is now in use at close to a hundred installations.The guiding principles of the design of C classes were simplicity both in concept and in implementation. Smooth blending of the new facilities into the existing C programming environment was considered essential. Classes can be used without sacrificing the run‐time efficiency expected from C. This paper presents some of the reasons why C classes are what they are.