An important part of any map is the sky. In Source, the sky is called a SkyBox, and it helps give the illusion that you're standing in a world that is bigger than what is really there. There are two types of Skyboxes available in Source: the 2D SkyBox and the 3D SkyBox. Let's have a look at them.
The 2D SkyBox is comprised of six images displayed on an infinitely large cube around the outside of the level. Let's set up a quick 2D SkyBox in our current map and we will see how it works. Select the four vertical walls of your giant box and using the Clipping Tool, cut them in half horizontally as shown in the following figure:
Select the ceiling and the top four sections of the walls you just clipped. We're going to apply a special texture to these brushes in order to form our SkyBox.
Select the texture tool from the tools palette, or hit Shift + A. The texture tool looks like a green, red, and blue cube.
Once the texture tool appears, hit the Browse... button to bring...