From Wikipedia

Extensible Markup Language (XML) is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification[4] produced by the W3C, and several other related specifications, all gratis open standards.[5]

What are the historical reasons for calling the shorthand XML rather then the more natural EML?

  • 14
    If it was EML someone would be asking why not XML.
    – M.Sameer
    Commented Jun 30, 2011 at 11:37
  • You would have to ask James Clark. He's the guy who coined it. en.wikipedia.org/wiki/James_Clark_(XML_expert) Commented Jun 30, 2011 at 14:08
  • Wasn't "EML" already a file format used by Microsoft Outlook?
    – user16764
    Commented Jun 30, 2011 at 15:42
  • 11
    It was the 90s, and everything cool had an X in it.
    – Andy
    Commented Nov 1, 2015 at 15:49
  • I fondly remember the 6502 microprocessor, the only processor that had an EOR instruction.
    – Mr Lister
    Commented Apr 3, 2016 at 13:05

7 Answers 7


I took a very pleasant tour through W3C, Google, and Wikipedia and finally found the answer: an annotated XML spec where we find an excerpt of an email from the inventor of the name, James Clark, an email from chairman Jon Bosak who suggested to use the X letter, and some other ideas for names and the final votes:

Votes | Acronym | Full Name
5     | XML     | Extensible Markup Language
4     | MAGMA   | Minimal Architecture for Generalized Markup Applications
3     | SLIM    | Structured Language for Internet Markup
1     | MGML    | Minimal Generalized Markup Language

This is Jon Bosak's reply to James Clark's suggestion to name it "Extensible Markup Language" and gave birth to the acronym:

In my opinion, the U-combinations won't fly, but if we allow "X" to stand for "extensible", then I could live with (and even come to love) XML as an acronym for "extensible markup language", and I hereby now throw it into the list of current proposals.

(Emphasis mine)

Some bonus - from some old reports of the XML Special Interest Group that I found while looking for some original quote that could answer the question:

M.15 Should the spec refer to XML as "The Extensible Markup Language" or as "Extensible Markup Language" without a definite article (e.g. in the first sentence)?

The WG elected to give no guidance to the editors on this issue (in the full expectation that the result would depend on which editor touched the file last).

Rationale: after several minutes' discussion and increasing hilarity, no consensus had been reached, but the end of the allotted time for the conference call had.

  • 9
    Really? I kinda like MAGMA. Pity it lost. Also, +1 for digging and finding that email where the discussion originated!! Commented Jun 30, 2011 at 14:22
  • 2
    Very nice detective work! Good to see the other options, and interesting they didn't even consider EML.
    – Hugo
    Commented Jun 30, 2011 at 16:14
  • 1
    FWFD - I agree. MAGMA's kinda cool.
    – Shauna
    Commented Jun 30, 2011 at 18:51
  • 6
    XML was a product of the mid-nineties, when seemingly everything had to be "extreme"... Commented Jun 30, 2011 at 20:19

Because "X" is just so much cooler than "E".

Also presumably to avoid confusion with things like Extensible ML.

  • Did Extensible ML exist before XML?
    – Raynos
    Commented Jun 30, 2011 at 10:36
  • 2
    I'm not sure. EML seems to have pre-deceased XML on the internet. ML, which EML is based on, dates from the seventies, so it's entirely possible. Then again, SGML, which XML is a subset of, has even earlier roots.
    – Scott
    Commented Jun 30, 2011 at 10:45
  • That is similar to the aspx acronym, except that in XML, the x stands for something.
    – StuperUser
    Commented Jun 30, 2011 at 10:45
  • 2
    The earliest reference I see to "Extensible ML" is a 1999 paper, whereas the first draft of XML was in 1996. However, "Extended ML" is earlier and I found it abbreviated to EML in 1991.
    – Hugo
    Commented Jun 30, 2011 at 11:02
  • Now @Andreas_D has provided the definitive answer, it's interesting to see they didn't even consider EML.
    – Hugo
    Commented Jun 30, 2011 at 16:18

From the book Computers & Society

The X in XML

"XML is both a boon and a threat to the web dream."

The X in XML stands for eXtensible. This signifies that XML is open. It is open in the sense that it allows growth. Note that the abbreviation is XML and not EML. There is something daring, attractive, exciting about that X. X stands for many things, depending on the culture of the society in which it appears! The two most obvious things denoted are

  • X-rated stuff: films, books... intended for adults only!
  • X-roads: where choices in direction can be made.

Since the X in XML is pronounced "ex" and may sometimes even be written that way, then we have an opportunity of charting other potential meanings. For example, to say that "she is his ex" implies something in the past. This is not the intention of the X in XML. But we can make it so. We can think of XML poetically as X-ML to refer to the SGML that preceded it.

And we can use exactly the same interpretation for XHTML.

  • 4
    Although nice, that's mere speculation and humour.
    – Raynos
    Commented Jun 30, 2011 at 10:43
  • True. I couldn't find any definitive answers, and found similar answers in other XML books. I don't see any relevant use of EML before XML.
    – Hugo
    Commented Jun 30, 2011 at 10:52
  • The earliest reference I see to "Extensible ML" is a 1999 paper, whereas the first draft of XML was in 1996. However, "Extended ML" is earlier and I found it abbreviated to EML in 1991. So perhaps EML was really 'taken' so they used XML instead.
    – Hugo
    Commented Jun 30, 2011 at 11:03

This is a question that maybe is better asked on http://english.stackexchange.com, generally, the english speaking world tends to use the X creatively, like X as 'cross' as in X-ing. In this case, it seems, 'X' represents the first Syllable ('eeeks'), not the first letter.


Speculating on the answer given by Andreas_D:

but if we allow "X" to stand for "extensible"

Notice that it seems like there's a bit of reverse engineering of the acronym here... we've got an X, what should it stand for??

It strikes me that eXtreme sports, the X games, etc were all very popular around 1995 and that the X in XML is probably a play towards that. Maybe someone hoped for eXtreme Markup Language?

  • 2
    See also DirectX, ActiveX, XBox
    – Andy
    Commented Nov 1, 2015 at 16:14

I've always thought of the X in XML as indicating that you can make any kind of markup language you want with XML. Back when it was new, that was one of the biggest things everyone talked about.

  • How does X indicate that I can make other languages with this one, any more than, say, Q or J? Commented Jun 30, 2011 at 14:23
  • 2
    Because 'X' used in this context can be shorthand for 'fill in the blank.' A variable, as it were.
    – Robert S.
    Commented Jun 30, 2011 at 14:36
  • There are many other letters that can be used as variables besides X. Commented Jun 30, 2011 at 14:45
  • 1
    Yeah, so? That's not the norm outside of mathematics. X has been used to define the unknown for quite some time. Other letters, not so much. C.f. X-ray, Generation X, etc.
    – Robert S.
    Commented Jun 30, 2011 at 15:30
  • 2
    X-ray is a good example: "In the paper he [discover Wilhelm Röntgen] admitted that he did not know the precise nature of these new rays. He chose to name them "X-rays," since "X" is the mathematical symbol for the unknown." discoveriesinmedicine.com/To-Z/X-ray.html
    – Hugo
    Commented Jun 30, 2011 at 16:23

X is pronounced ex, and it's fairly common in acronyms to use X for words starting with ex-.

This would be better asked on English.SE

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