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It's why in a lot of games most walls are only visible from one side. The graphics system checks which direction each triangle is wound before it begins filling them in or texturing them.

This saves on computation, since around half the geometry in the world is known to not be visible and is not processed, even before doing any depth checks.

Polygon winding order. That is, when projected by a matrix, are the points of the triangles presented in clockwise or counter-clockwise order.

Which took me about 3 hours of wondering why my winding order was backwards to figure out -_-