Primitive geometries must implement the Primitive interface. ISO19107 describes the primitives as a geometric object that couldn't be descomposed in another primitive on the system. Solid is a primitive too, beacuse the diferent sides of a solid haven't got sense by themselves.

The structure of primitives on gvSIG is shown in the next figure:

Here there's an important difference between ISO19107 and the model, because a Curve isn't composed by elements with smaller granularity than it, but is the own geometry which choose how to implement itself. In the Surface case is the same.