Abstract: A window manager for a windowing system includes a window server to command the display on screen of first windows reserved by client programs of the system. The window manager manages the disposition and the circulation of first windows within a second window reserved by the system. The server holds in memory a tree structure of windowing data on the basis of which the window server communicates with the screen window manager and with the client programs. This structure has a root defining the second window and nodes defining corresponding first windows. The window manager manages in the root a first list of identifiers of first windows and detects in each node the presence or the absence of control data. If control data is present in a node defining a particular first window, the window manager manages the disposition and the circulation of the first windows on the basis of second window data under the control of a client program.