Unofficially, the PHIGS Extension to X. Officially, PEX was not an acronym.

This is http://www.jch.com/~jch/vrml/PEXTimes.html
See also, http://www.jch.com/~jch/vrml/timeline.html

Q2 1963 SKETCHPAD paper published, Ivan Sutherland
Oct 1971 GINO paper published, P.A. Woodsford.
Apr 1974 NBS (National Bureau of Standards) & ACM SIGGRAPH create GSPC
May 1976 Seillac (sy ack) France, International Computer Graphics Standards Big Bang
??? 1977 CORE published by GSPC Graphics Standards Planning Committee ( and 1979)
Feb 1979 GKS/CORE reconciliation meetings
??? 1979 E&S introduces PS300; Computervision introduces GPU both 3D traversers.
Q4? 1980 SIGGRAPH CORE group reorganizes as ANSC X3H3.1 (PHIGS)
Feb 1985 PHIGS first public review
Aug 1985 GKS becomes first ISO Graphics Standard
Dec 1985 X10R1 Release, fixed for long word alignment for RT.
Apr 1986 Transactions on Graphics Article, The X Window System. Bob Scheifler & Jim Gettys
Jul 1986 X11 Protocol Document released for public review
Nov 1986 PHIGS+ Ad hoc group gets together under direction of Andries van Dam.
Dec 1986 X10R4 released, the last revision of X10.
Jan 1987 X conference 1st. 3D BOF Branko Gerovac, Major X11 announcement.
Jun 1987 Cognition ships first commercial X application and X server for the PC.
Jun 1987 X11 3D (X3D) Workshop. Bert Herzog. Digital & Sun propose 3D extension
Aug 1987 XPHIGS 1.0, Randi Rost Document Editor, 2 DEC, 2 Apollo, 2 Sun, 1HP
Sep 1987 X11R1 released, developed by Susan Angabrant & Co at Digital.
Oct 1987 PHIGS DIS, Draft International Standard, (technical content frozen)
Oct 1987 XPI (XPHIGS/PEX) Version 2.0, nearly complete, still old resources.
Dec 1987 PEX 3.0 Specification. Major rewrite to bring PEX to current form.
Jan 1988 X conference 2nd, Kresge Auditorium. First X Consortium Meeting.
Mar 1988 X11R2, Lots and Lots of bug fixes, first widely used X11 Xt.
Apr 1988 GM/EDS PHIGS 1.0, major enhancement is "post to view" or PTV.
Jan 1988 PHIGS+ 3.0, this is submitted to ISO to become PHIGS-PLUS.
Aug 1988 SIGGRAPH 1st PEX Demo, University of Michigan, Spencer Thomas. DEC, HP & Sun participate.
Oct 1988 X11R3 released, Xt "standard," much improved imake utility.
Dec 1988 PHIGS becomes ANSI Standard. X3.144-1988, soon followed by ISO 9592
Dec 1988 OSF announces Motif as the result of an RFT (OSF founded in April)
Dec 1988 PEX-SI Kick-off, Marty Hess manages SI group at Sun.
Jan 1989 X conference 3rd (First one in Boston).
Mar 1989 AVS 1.0 released by Stellar Dave Kamins, Jeff Vroom
Jun 1989 PEX-SI special release for SIGGRAPH.
Jul 1989 OSF/Motif 1.0 released to industry.
Aug 1989 SIGGRAPH 89 UNICAD, based on PEX-SI code. HP & Sun. DEC has product.
Dec 1989 PEX 4.0 Specification, Hardenbergh & Plunkett make it to front cover.
Dec 1989 X11R4 Released, ddx performance rewrite, ICCCM, Compound Text, XLFD.
Dec 1989 AVS 2.0 released.
Jan 1990 X conference 4th. Digital announces PEX product.
Apr 1990 AVS 2.0 on Titan (Ardent, now Kubota), portable AVS (sold to DEC,E&S,Convex).
Jun 1990 PEX-SI Alpha release.
Aug 1990 SIGGRAPH, (low key for PEX) E&S demos PEX product, GPC/PLB
Sep 1990 PEX 5.0 Specification, Sally Barry Document Editor
Oct 1990 PEX-SI Beta release.
Nov 1990 PEXIC Organizational meeting, New York. PEXIM & PEXtk announced
Jan 1991 X conference 5th, PEXIM, PEX-SI papers & courses.
Feb 1991 PEX-SI released from Sun to MIT.
Feb 1991 PHIGS-PLUS DIS, Tom Gaskins Editor.
Apr 1991 Toby Howard et al publish first book on PHIGS.
Apr 1991 AVS 3.0 ships on first Platform
May 1991 PHIGS Immediate Mode Ad Hoc group gets together.
Aug 1991 SIGGRAPH PEX-IC Interoperability demo, Convex, DEC, E&S, HP, IBM, Kubota,
 ShoGraphics, Stardent, Sun, Tek.
Sep 1991 Kickoff of PEXlib effort, Jeff Stevenson heads MIT standards effort.
Sep 1991 SGI announces OpenGL.
Oct 1991 X11R5 released with PEX, the Font Server and I18N support.
Jan 1992 X conference 6th, PEXlib announcement, PEX Programming tutorial.
Feb 1992 PEX Interoperability Center opens at Convex, Dave Holt as president.
Mar 1992 Tom Gaskins PHIGS Programming Manual published by O'Reilly.
May 1992 PEX 5.1 Specification, Cheryl Huntington & Paula Womack.
Q2? 1992 PHIGS-PLUS IS text released from ISO.
Jun 1992 Jeff Stevenson chosen to be pex-spec CA, Karl Schultz to head PEXlib 6.0 effort.
Aug 1992 PEXlib 5.1 Specification, Jeff Stevenson.
Jul 1992 SIGGRAPH demo based on PEX 5.1, tutorial, the great debate. SGI's Inventor
Sep 1992 EG92, Eurographics PEXlib Programming Tutorial.
Nov 1992 PEXlib released from MIT.
Dec 1992 Tom Gaskins' PEXlib book.
Jan 1993 X conference 7th, PEXlib tutorial.
Mar 1993 Common Open Systems Environment group endorses PEXlib for UNIX.
Aug 1993 SIGGRAPH full week labs. HP, Sun & Kubota car racing. Finally a sexy PEX demo.
Oct 1993 CAGE PEX extension specification published by HP, IBM and Sun.
??? 1994 PEX 5.2 Protocol Specification and PEXlib to match.
Dec 1996 X Consortium closes doors.

The infamous PEX buttons - every SIGGRAPH, a new button came from nowhere.
With the help of Dick Coulter and Allen Galotta, here is the list of PEX
buttons:

1989 - Safe PEX                      (Boston)
1990 - Hot, steamy PEX               (Dallas)
1991 - Silicone PEX or Bust          (Las Vegas)
1992 - phone PEX (1-900-good-pex)    (Chicago)
1993 - PEX starved?                  (Anaheim)
1994 - ORL PEX                       (Orlando)
1995 - RIP PEX                       (LA)

If you have any additions or corrections, don't hesitate: mailto:jch@jch.com