This is important, and like non-manifold geometry it's easy to miss.
The surface normal (sometimes called a face normal) is the directional vector perpendicular to the surface of a 3D model. Every face has its own surface normal, and it should be facing outward, away from the model's surface.
However, this doesn't always prove to be the case. During the modeling process, a face's surface normal can accidentally be reversed by an extrusion, or through the use of other common modeling tools.
When the surface normal is reversed, the normal vector points toward the interior of the model instead of away from it.
Fixing surface normals:
Thankfully, it's quite easy to fix a surface normal problem once you know it exists. Surface normals are not viewable by default, so you'll most likely need to change some display settings to spot any issues.
- In Maya, the easiest way to achieve this is to go up to View -> Lighting and deselect Two-sided lighting. When two-sided lighting is disabled, any faces with their surface normals reversed will be completely black.
- Select these faces, go to the Polygons menu set, and select Normals -> Reverse.
- Checking your surface normals from time to time is a good habit to get into regardless of whether you ever plan on printing your models.
The instructions are slightly different in every 3D software package, but should be readily available in your application help files.