Reverse Normals
Modify > Reverse Normals
 

Description:

      The Reverse Normals (Modify > Reverse Normals) command will reverse the direction of each selected face's normal vector.  If you are in object selection mode, all the normal vectors on the selected objects will be reversed.  Normal vectors determine which direction a face considers to be pointing "out".


Notes:

-Reverse Normals can be performed on the selected faces of multiple objects simultaneously.

-You can set an object to display its normals by choosing Display > Object Display Mode > Show Normals.

- Several operations, including Shell, Booleans, and Extrude Along Normals require that an object's normals be aligned in a certain way (usually all pointing out).