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The Skybox is the Image around your map - This tutorial is 100% Necessary if you are making an outdoor map.
You will be taught how to use the entity; "Func_Door_Rotating". Pretty easy :)
In this tutorial, you will be taught how to make the blue areas on textures e.g. fences, air-ducts and so on, and so fourth, invisible.
These are the standard Measurements, taken from cs_mansion which is practically older than CS. It includes; doors, walls and windows. - Very basic Mapping...
In this tutorial, you will be taught how to clip objects, using the Clipping tool.