Open and Closed NURBS

NURBS can be open or closed. When closed, the minimum and maximum boundaries are joined by segments or patches.

Open curve	Closed curve

A surface can be open in both U and V like a grid, closed in both like a torus, or open in one and closed in the other like a tube.

Open in U and V.	Open in U, closed in V.
Closed in U, open in V.	Closed in U and V.