Normals > Reverse

Reverses the normals on the selected polygons. You can also specify whether or not the user-defined normals get reversed.

Normals > Reverse >

Specifies what elements have their normals reversed.

Selected faces: Maya reverses the normals on the selected faces. This is the default setting.
Selected faces, then extract: Maya reverses the normals on the selected faces and then extracts, or splits the vertices.

NoteThe Reverse option can result in non-manifold geometry, where polygons that share an edge have reversed normals. Some tools cannot work with non-manifold geometry. See Clean up, simplify, or validate meshes.
To avoid creating non-manifold geometry, use the Select faces, then extract option. This option splits vertices so as not to create non-manifold geometry.
All faces in the shell: Maya reverses the normals on all (selected or unselected) faces in the shell.

Specifies whether or not user-defined normals are affected by the Reverse operation.

Preserve user normals direction: Specifies that User-defined normals are not reversed when using the Reverse feature. The default setting is off.
Reverse user normals: Specifies that user-defined normals are reversed when using the Reverse feature. This is the default setting.