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To this day on Linux configs and startup scripts for the GUI system are stored in /etc/X11. I remember this name appearing alone, or alongside others - XFree86, X.org, X - for as long as I had contact with UNIX/Linux systems. I'd guess '11' is some version number, but why would this particular one be so prevalent? Why do we never hear of X10 or X12 window system while X11 survived a couple decades?

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    I recall reading sometime there once was a W, which stood for windowing system or something like that. Someone later decided to call their windowing system after the next letter alphabetically, just like the programming languages B, C, D etc... – Wilson Jun 7 '17 at 8:33
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    Why do we never hear of X10 [...]? There is still support for X10 mouse reporting mode: stackoverflow.com/questions/5966903/… – ninjalj Jun 7 '17 at 15:33
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    X10 was somewhat popular. My first college course used Sun 3/50 workstations with X10R4. (Diskless workstations with 4MB of RAM, X10, and Emacs, swapping over NFS... ouch.) – fadden Jun 7 '17 at 16:27
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    @fadden swapping over NFS... I guess you have seen this interesting effect of the screen being updated, by line, top to bottom - with the timing of network packages coming in, each delivering a random, but small amount of screen lines? Fascinating. – Volker Siegel Jun 8 '17 at 3:17
  • @VolkerSiegel: I set up a Sun SLC to run off another ancient Sun, NetBSD, swap over NFS too. But it wasn't importing 'foreign' X display, just the filesystem with X executables. Still wasn't entirely fast, to put things mildly. – SF. Jun 17 '17 at 14:04
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The short version is that X11 was the first widely-disseminated version, and it turned out to be good enough to remain as-is for thirty years.

X is the X Window System, which at its core is a protocol; the number identifies the version of the protocol. X1 was released in 1984 inside MIT, and quickly evolved to X9 in 1985; an external port to RT/PC required a protocol change, so X10 followed shortly thereafter. At this point X “escaped” to other organisations, but its developers really wanted a more device-neutral re-design. DEC contributed to this, and released X11 in 1987. (This is a quick summary of Wikipedia’s history of X.)

The protocol itself is stable, because it’s device-neutral and extensible. As a result, there has been no absolute need for a new protocol version (X12). The wide dissemination of X11 also means it’s harder to introduce a new protocol version, and instead, new features are introduced as extensions (sometimes jumping through hoops).

There are a number of limitations in X11, and there’s even an “X12” page (which isn’t about an actual X12 project, just a list of what it would have to do and relevant resources).

The successor of course is likely to be Wayland, which is developed by X developers.

As pointed out by Wilson, the letter X was picked as a follow-on to W, which was the windowing system of the V operating system.

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    V-W-X... Now I'm just wondering, "Why V?" – Izkata Jun 7 '17 at 17:55
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    Could be a reference to the VAX machines it was made on, to verex, or perhaps even a play on unix system IV. – user1090729 Jun 7 '17 at 21:36
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    Or maybe they started with "U", which caused too much confusion with the homophone "you", so they incremented to "V". – HopelessN00b Jun 8 '17 at 6:20
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    V-W-X-W-ayland... – a CVn Jun 8 '17 at 15:12
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    @cat it’s the other way round, XWayland is an X server running inside Wayland (so X clients can run in a Wayland environment). – Stephen Kitt Jun 8 '17 at 17:28

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