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A window manager is software that controls the placement and appearance of application windows under the X Window System, a graphical user interface on Unix systems that enables a user to interact with a number of application programs simultaneously. Each one typically has its own independent window, and when a window manager is available, interaction between the X server and its clients is redirected through the window manager.
Unlike the Microsoft Windows and Apple Macintosh platforms, which have historically provided a vendor-controlled, fixed set of ways to control how windows and panes display on a screen, and how the user may interact with them, the X Window System allows the user to choose between various third-party window managers. Window managers differ from one another in several ways, including:
Alternative shells for Microsoft Windows have also emerged. For example, LiteStep can replace the user interface on Windows 95, 98, or NT with an Afterstep style. OS/2 ships with Presentation Manager as the default shell, but third party sources can supply alternatives.