X Consortium
The vendor club that looked after X11 from 1988 to 1996 and lent the MIT License one of its names.
Once X11 became an industry standard, no single vendor wanted to own it, and none wanted a competitor to own it either. The answer was the MIT X Consortium, founded in 1988 with Bob Scheifler as director and membership fees from DEC, HP, Sun, IBM and a few dozen others. It wrote the specifications, maintained the sample implementation and shipped releases X11R2 through X11R6, including ICCCM, XIE and most of the extensions still found in every server.
In 1993 it left MIT to become X Consortium, Inc., and at the end of 1996 it was wound up. The rights went to The Open Group, which promptly tried to charge for X11R6.4 and just as promptly backed down. The legacy now sits with the X.Org Foundation and the successors to XFree86. So does the licence text: the "X11 License" is, for practical purposes, the MIT License with the consortium's name in it.