A client can request the creation of a window. More precisely, it can request the creation of a subwindow of an existing window. As a result, the windows created by clients are arranged in a tree (a hierarchy). The root of this tree is the root window, which is a special window created automatically by the server at startup. All other windows are directly or indirectly subwindows of the root window. The top-level windows are the direct subwindows of the root window. Visibly, the root window is as large as the virtual desktop, and lies behind all other windows.