[Stiglr]

Welcome to the world of "normals".

Many gaming systems only "display" (or assume) one-sided surfaces, so you must make sure that the displaying face (normal) is turned outward. If you create an object that has normals turned the opposite way, you can easily change it on an object basis or on a surface-by-surface basis. In the Surface menu, you should be able to find "Flip Normal" commands to use to rectify the situation.

AC3D also has a viewport display check box for normals that, when checked, displays a pink line extending away from the normal side, so you can see which way a surface faces. This works well for close-up work, but for a complex high-poly file, it creates a "porcupine" effect that is visually overwhelming.