Select NURBS object. > Modify panel > Create Surfaces rollout > Dependent Surfaces group box > Cap
Select NURBS object. > Modify panel > NURBS toolbox > Create Cap Surface button
This command creates a surface that caps a closed curve or the edge of a closed surface. Caps are especially useful with extruded surfaces.

Capping an extrude surface
To create a cap surface:
In a NURBS object, turn on Cap.
Move the mouse over the closed curve or the closed edge of a closed surface.
If the cap can be created, the curve or edge is highlighted in blue.
Click the highlighted curve or edge.

The Flip Normals control lets you flip the surface normals at creation time. (After creation, you can flip normals using controls on the Surface Common rollout.)
While a cap surface sub-object is selected, a rollout with cap surface controls is displayed at the bottom of the Modify panel.
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