The basic idea is that the surface admits a differentiable structure, so that it is possible to do calculus on the surface. For most points, there will be a well-defined tangent plane, though this does not rule out self-intersection and some other degenerate behavior.

**See also:**

- Polyhedral surfaces

* 8/13/94 dpvc@geom.umn.edu -- *

*The Geometry Center*