Unify Normals
Modify > Unify Normals
 

Description:

      The Unify Normals (Modify > Unify Normals) command will attempt to align each selected face's normal vector to those of the faces around it, or, if in object selection mode, it will attempt to align all of the object's face's normal vectors to each other. On unusual geometries a complete unification of normals may be impossible, but in the majority of cases this will have the effect of finding an "interior" and "exterior" of the object, with all normals either facing in or facing out.


Notes:

- Unify 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, Boolean, and Extrude Along Normals require that an object's normals be aligned in a certain way (usually all pointing out).