A reusable model of a three-dimensional object is created on a computer system. The reusable model includes model objects and internal constraints on the model objects. The internal constraints maintain the structure of the reusable model with respect to the three-dimensional object. The reusable model can have inputs that accept directed and non-directed external constraints with models using the reusable model. The inputs define the direct methods for modifying the structure of the reusable model. The reusable model can have outputs that have directed constraints with models using the reusable model. The outputs provide a direct method for using the structure of the reusable model to modify the structure of the model using the reusable model. Nondirected constraints may be associated between the model using the reusable model and the reusable model to position the reusable model in the model.