X-windows is a system that divides the labor up quite a bit. An X-windows application draws and controls only the area "inside" the window, the so-called client area. A totally separate program, called a "window manager" paints the borders and controls (such as resizing edges, maximize, minimize, and close buttons). You can have only one window manager program per X server, and that window manager provides these window "decorations" on behalf of all windows.

The window manager and the X-application communicate in standard ways. For example, when you resize a window by dragging an edge, all the work is done by the window manager program until you release the edge; then the window manager tells the X application about it by sending it a resize message. The application then must repaint itself in the new client area. (This is a bit theoretical: Some window managers send repaint messages while you drag the edge, so the application repaints several times, but you get the idea.) The application is also able to tell the window manager certain things, such as whether or not the window may be resized.

The division of labor has some distinct advantages.