X11 is Version 11 of the X Window System, commonly used on , , and systems to provide the lower level graphics, input, and window management API's that higher level desktops such as GNOME and KDE build upon.

The primary open source implementation is now published by the X.Org Foundation on the www.x.org web site. Version 11 was first released in 1987, and while many subsequent releases have occurred since, they have maintained backwards compatibility with the Version 11 protocols and APIs.

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