A window manager is a program that acts as a graphical go-between for a user and an OS. It provides a GUI for the OS. Some OSes incorporate the window manager into their own internal code, but many do not for reasons of efficiency. Some OSes partially make the division. Some common true window managers include CDE (Common Desktop Environment), OpenWindows, Motif, FVWM, GNOME, and Enlightenment. Some common hybrid window managers with OS extensions include Windows ME, Windows 98, Windows 95, Windows 3.1, OS/2 and GEOS.