Because of the underlying representation of NURBS surfaces, there are some things they cannot model:

• Topologies that are not equivalent to a rectangular sheet.

  Spheres, cones, tori, and triangles can all be built from sheets by attaching or collapsing sides. But more complex shapes, for example a star shape, cannot be represented with a simple NURBS surface. To get a complex surface outline, you must use a trimmed surface or a network or collection of four-sided surfaces.

• Holes.

  To create a hole in a surface, use a trimmed surface.

• Surfaces that cannot be mapped with regular U and V coordinates.

  For example, you can model the shape of a Mobius strip, but the surface will have a seam.