2.2 What are X Windows?

An X server controls a bitmapped screen. In order to make it easier to view and control many different tasks at the same time, this screen can be divided up into smaller areas called windows. A window is a rectangular area that works in several ways like a miniature screen. Windows on the screen can be arranged so they all are visible or so they cover each other completely or partially. A window may be any size greater than zero.

Each window (on a screen running X) can be involved in a different activity, and the windows currently in use are placed so they are at least partially visible. The window manager lets you move a different window to the top when necessary or rearrange the size and position of the windows.

What you may not have realized is that some of these windows, asuch as the ones created by the mail handler xmh, are made up of many layered windows of various sizes. The scrollbars, titlebar, command buttons, and other features of the user interface are actually separate windows that provide information to the user or allow for input providing convenient control, as shown in Figure 2-4. There is more here than meets the eye.