'Window system X'  announced at MIT. Versions 1-6 were monochrome only, and ran on DEC VS100's displays connected to VAXen and VAXstations 1 and 2. 
Versions 8-10 dealt with color, for the VAXstation II/GPX. X10 is the first version that saw widespread availability and use on many vendor's systems.
Version 11 was redesign for higher performance, more window management styles, extensibility and better graphics capability.