Autodesk AUTOCAD 2006 Command Reference Manual page 1143

Shell
Shelling creates a hollow, thin wall with a specified thickness. You can specify
a constant wall thickness for all the faces. You can also exclude faces from the
shell by selecting them. A 3D solid can have only one shell. New faces are
created by offsetting existing ones outside their original positions.
Select a 3D solid: Select an object
Remove faces or [Undo/Add]: Select one or more faces or enter an option
The descriptions of the Undo, Remove, Add, and All options match the
descriptions of the corresponding options under Extrude. After you select faces
or enter an option, the following prompt is displayed:
Remove faces or [Undo/Add/ALL]: Select a face (1), enter an option, or press ENTER
Enter the shell offset distance: Specify a distance
1
face selected shell offset=0.5 shell offset=-0.5
Specifying a positive value creates a shell to the inside perimeter of the solid;
specifying a negative value creates a shell to the outside perimeter of the solid.
Clean
Removes shared edges or vertices having the same surface or curve definition
on either side of the edge or vertex. Removes all redundant edges and vertices,
imprinted as well as unused geometry.
Select a 3D solid: Select an object (1)
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