GtkRadiant/The CSG Tools
The next set of important brush tools are called the CSG tools. CSG in this case stands for Constructive solid geometry which, in a nutshell allows you to cut and merge brushes, as well hollowing them. The tools are called CSG Subtract, CSG Merge and Hollow.
This tool allows you to "cut out" or "subtract" one brush with another. See the figure with the purple and olive brush. Modelers know this as a Boolean operation. To use CSG Subtract intersect two or more brushes, then select the brush you want to cut with, in the example, this was the olive one. Then press the first CSG Subtract button in the toolbar .
If you then delete the brush you cut with, you should now find that there's a hole in the other brush(es) where that brush was. If you select the brushes left, you will most likely see that they have split up into smaller brushes. This is because all brushes in the id tech 3 engine (and therefore in GtkRadiant) have to be convex.
This tool is very useful to make holes into brushes when placing doors etc. But be careful, if you use this with to complex objects, you might find yourself with an excessive number of brushes as result. So if you want to do arches or other round things, you're probably better off with patches and end caps.
This tools function is very simple, it merges multiple brushes into one brush. So to use it, select two or more brushes which align with each other on at least one side. Then press the CSG Merge button the in toolbar.
Generally, the reason why you only can merge aligning brushes is the same reason as for the subtract, id tech 3's inability to handle concave brushes. So a rule of thumb would be that any two brushes which when merged would become a concave brush cant be merged.
The Hollow tool allows you to (surprise) make a brush hollow. This replace the original brush with six brushes, making up the sides of the brush you had. The "side" brushes will be as thick as the grid size you have selected. This tool does however overlap the brushes in the corners, which should be avoided, so when using this tool, always fix the overlapping corners.
To use it, simply select the brush you want to hollow, make sure you have chosen the grid size you want, and click the hollow button in the toolbar.