Clients specify which kinds of events they want to be sent by setting an attribute of a window. For example, in order to redraw a window when its content has been destroyed, a client must receive the codice_16 events, which inform it that the window needs to be drawn again. The client will however be sent codice_16 events only if the client has previously stated its interest in these events, which is done by appropriately setting the "event mask" attribute of the window.