We use "x" to represent unknown quantities all the time. It appears in algebra (solve for x), but also in culture ,the letter "x" was used as an symbol of unknown, is it there any alternative for unknown as per usability perspective?

Is it the only representation of "x" the symbol of unknown?any other alternative for unknown representation in usability perspective?

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    Not sure if there are alternatives, but check this TED talk for the explanation of 'x': ted.com/talks/terry_moore_why_is_x_the_unknown
    – JonSpr
    Commented Jun 11, 2014 at 10:26
  • 2
    @JonSpr Neat background on the TED talk. --BlueBerry, aside from the question mark, I can't really think of much else for an alternative. I think the algebraic usage / what's mentioned in the TED talk is most likely the primary cause. There's also the general shape of X, crossroads, the rarity of the ex sound in language and a few small contributing factors leading up to the stigma of X as mysterious =] (not to mention...pirate maps!) Commented Jun 11, 2014 at 12:02
  • 2
    Someone has been reading io9! :) io9.com/… Commented Jun 11, 2014 at 15:00

3 Answers 3


The first part of your question, Why Does “X” Represent the Unknown?, can be answered quickly from this short TED talk.

In short, we use "x" to represent the unknown because the scholars in 11th century couldn't translate the arabic letter "Shin", denoting unknowns, to Spanish. In Arabic, Shin (ش‎) is read as /ʃ/, like the sound sh in shoe, but there is no such sound in Spanish. For this reason, they borrowed the Greek letter "Chi" (Χ, χ) instead, which was later transliterated to X.

The second part, about the alternatives for the unknown from the usability perspective, it's hard to answer without knowing the context. Some of the possible alternatives are:

  • Question mark, used in post-its, or notes when you need to draw charts/navigation schemes/sitemaps. Chart with questions

  • Dot or Dash, e.g. ---, ___, which you use to represent the "blank" to be filled in, which is itself an unknown. Fill in the Blank

  • Form elements input areas, e.g. text field, dropdown with placeholders ("please type", "please choose", etc.) are also unknowns in a way. enter image description here

  • Nice picture.. but didn't you just squish together the other two answers already here? Commented Jun 11, 2014 at 11:54
  • Hi @GaretClaborn, there's only 1 answer before my answer which doesn't cover what I wanted to say. I guess you mean there's also another comment to the question (which I admittedly overlooked when skimming through). Anyway, neither answered the question the way I intended to so here I am. Thanks for the notice. Commented Jun 11, 2014 at 12:04
  • Oh I see,... yeah ya'll both linked the same TED video. I guess I just don't quite get where you're coming from on placeholders or post-its representing the unknown for users (maybe for the developers). still, if you felt that was important, then fair enough. Commented Jun 11, 2014 at 12:19
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    A highly disputable explanation. x probably comes from Descartes. Cf. jeff560.tripod.com/variables.html (Search for “Descartes' use of z, y, x.”)
    – akater
    Commented Jun 11, 2014 at 18:06
  • Thanks a lot @Giulio Muscarello for suggesting the changes. I added a few of my own changes too on top of your suggestions, hence this comment. Commented Jun 11, 2014 at 18:47

I think your best alternative is the question mark "?", better than the "X" icon i guess.

As you can see by this google search, "?" is a better option than "X" to represent Unknown.

Google search for Unknown icon

  • 2
    The question mark however, often stands for 'help'. So this doesn't work in all contexts.
    – JonSpr
    Commented Jun 11, 2014 at 11:35
  • @JonSpr wouldn't you say the same for X ? While it can represent an open variable, it also means close to a lot of folks. (Plus, when you need the help feature, it is usually for things that are unknown to you) Commented Jun 11, 2014 at 11:52
  • The X for closing is actually the "cross" sign, not the letter X. They may look similarly though. Commented Jun 11, 2014 at 11:59

As some have noted in comments on the TED talk provided, and on the answer here referencing it, there is an alternative explanation which some take as more reliable. It is described in Alex Bellos book Alex's Adventures in Numberland, and roughly goes like this:

In La Géométrie Descartes introduces what has become standard algebraic notation. It is the first book that looks like a modern maths book, full of as, bs and cs and xs, ys and zs. It was Descartes’s decision to use lower-case letters from the beginning of the alphabet for known quantities, and lower-case letters from the end of the alphabet for the unknowns. When the book was being printed, however, the printer started to run out of letters. He enquired if it mattered if x, y or z was used. Descartes replied not, so the printer chose to concentrate on x since it is used less frequently in French than y or z. As a result, x became fixed in maths – and the wider culture – as the symbol for the unknown quantity. That is why paranormal happenings are classified in the X-Files and why Wilhelm Röntgen came up with the term X-ray. Were it not for issues of limited printing stock, the Y-factor could have become a phrase to describe intangible star quality and the African-American political leader might have gone by the name Malcolm Z.

See supporting evidence in the link Akater has provided.

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