Abstract
Computer aided design systems tend to work simply in terms of geometric entities. Any intelligence that could be implanted in the system would, however, need to deal with the function of the design being described. This requires the system to handle the geometry at a higher level. One approach is to use model spaces that hold groups of geometric entities and preserve some of the associations between them. A software system is described that works in terms of such spaces and allows hierarchies of spaces to be built up. The hierarchy, however, is not sufficient for a complete description of every design. There is a need to add extra constraints. The software allows these to be entered and automatically tries to maintain them as being true. The use of this approach is illustrated by the simulation of the motion of a mechanism from a high-speed packaging machine.