@aescling

X was developed jointly by MIT’s Project Athena and Digital Equipment Corporation, with contributions from many other companies. It was masterminded by Robert Scheifler and colleagues at MIT, though it owes some debt to the "W" windowing package developed by Paul Asente at Stanford.

There have been numerous research versions of X. Version 10, Release 4 (popularly known as X10.4), which was released in 1986, became the basis for several commercial products.
Development of most X10.4 products was curtailed, however, when it became apparent that Version 11 would not be compatible with it. Version 11, Release 1 became available in September 1987, Release 2 in March 1988, Release 3 in February 1989, Release 4 in January 1990, and Release 5 in August 1991.

Version 11 is a complete window programming package. It offers much more flexibility in the areas of supported display features, window manager styles, and support for multiple screens and provides better performance than X Version 10. It is fully extensible. But just as important, the X11 subroutine library (Xlib) is expected to be stable for several years and to be at least a de facto industry standard. That means that programs written with this library will not need major revisions because of software updates. While there may be additions to this library, there will not be incompatible changes to it.