X11R6 initially under the same license. The X11 License and the X11R6 "MIT License" chosen for ncurses and continued to be used after copyright transfer to the Free Software Foundation both include the following clause, absent in the Expat License:

As of 2020, the successor to the X Window System is the X.Org Server, which is licensed under what is effectively the common MIT license, according to the X.org licensing page:

The X.Org Foundation has chosen the following format of the MIT License as the preferred format for code included in the X Window System distribution. This is a slight variant of the common MIT license form published by the Open Source Initiative

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