This paper describes the GRaphic INput subsystem (GRIN) of an experimental volume modeling system called the Geometric Design Processor (GDP) developed at the IBM Thomas J. Watson Research Center. Sitting at an interactive graphic workstation, a mechanical designer generates computer volume models of complex physical objects and mechanisms built up from primitive volumes, e.g., cuboids, cylinders, swept surfaces, etc., entered at any orientation in 3-dimensional space. Objects are represented in the model as polyhedral approximations. The central issue is the provision of an efficient, natural means for a mechnical designer to enter and interact with these models.