345: COMPUTER GRAPHICS PROCESSING AND SELECTIVE VISUAL DISPLAY SYSTEMS (link)
W Window System Window system for the Stanford V kernel.
X Window System X9 (c) September 1985 Massachussets Institute of Technology & Digital Equipment Corporation, released under the MIT License. Networked access to graphics windows.
8-1/2, the Plan 9 Window System Rob Pike, AT&T Bell Laboratories, Paper: USENIX Conference Proceedings, Summer 1991, pp 257-266 -- put virtual graphics windows into the filesystem; windows support the basic graphics device interface so you can run a window system instance insude a window of another instance of the window system.