I define Unit as a unit of length measurement close to the inch and rounded for ease of calculation. Of course, this is true for most games, but the developer can change the proportions depending on what they are doing.

Example shader persets for Quake 3 mapping

Basic tutorial that covers how to use TrenchBroom for Half-Life (Counter-Strike 1.6) mapping, initial setup and some basic features.

How to fix most common probles with q3map2

List of most common warnings and errors you can get while compiling a map.

This explains how to create your own skybox using Terragen.