also be a pixmap, a number, a list of objects, etc. Selections and drag-and-drop are active mechanisms: after the user selects data in a window, the client handling the window must actively support a protocol for transferring that data to the application requesting it. Cut buffers, by contrast, provide a passive mechanism: when the user selects some text, its content is transferred to a cut buffer, where it remains even if the application handling the window terminates and the window is destroyed. Window manager A window manager is a program that controls the general appearance of windows and other graphical elements of the graphical user interface. Differences in the look of X Window System in different installations stem mainly from the use of different window managers or from different configurations of the window manager. The window manager takes care of deciding the position of windows, placing the decorative border around them, handling icons, handling mouse clicks outside windows (on the “background”), handling certain keystrokes, etc. From the point of view of the X server, the window manager operates as a client just like any other client. The initial position and the decorative borders around windows are handled by the window manager using the following requests: an application can request the server not to satisfy requests of mapping (showing) subwindows of a given window, and to be sent an event instead; an application can request changing the parent of a window. The window manager uses the first request to intercept any request for mapping top-level windows (children of the root window). Whenever another application requests the mapping of a top-level window, the server does not do it but sends an event to the window manager instead. Most window managers reparent the window: they create a larger top-level window (called the frame window) and reparent the original window as a child of it. Graphically, this corresponds to placing the original window inside the frame window. The space of the frame window that is not taken by the original window is used for the decorative frame around the window (the “border” and the “title bar”). The window manager manages mouse clicks in the frame window. This allows, for example, a user to move or resize the window by clicking and dragging on the border or on the title bar. The window manager also handles icons and related visual elements of the graphical user