Twosided
From The DarkMod Wiki
Surfaces with the twosided keyword in their shader simply get their data doubled, so they get rendered twice, once from front and once from back. Here is a visual explanation.
The surfaces are doubled by FinishSurfaces() during model load. This means that if you have a model with a two-sided surface and a skin that switches to a one-sided surface, the surface will still be two-sided (as skin changes do not change the model). Likewise, if the surface is one-sided, but the skin surface is two-sided, the model will stay one-sided. This is tracked as bug #2297.
