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Why unknowns are usually denoted by "X" ?

More precisely: is this answer really a serious answer or might be a 1 April joke ? Let me sketch it. But please watch it, it is really fun and cool and < 6 min. (Are there any alternative versions?)

Answer - because in Spanish there is no sound for "sh" :)

Spain was under Arabic influence for a quite a long and played as bridge translating Arabic knowledge to Europe. In Arabic there was some special word for unknowns similar to English "something", but it contained sounds "sh" which was not present in Spanish. It was somehow substituted by "K" and later under further translation to Latin it became "X"...

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    $\begingroup$ According to jeff560.tripod.com/variables.html, Florian Cajori's History of Mathematical Notations says there is no evidence for this theory. $\endgroup$
    – Henry Cohn
    Commented Aug 5, 2012 at 18:16
  • $\begingroup$ @Henry that should be an answer not comment. $\endgroup$
    – Alexander Chervov
    Commented Aug 5, 2012 at 18:45
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    $\begingroup$ I was under the impression that, at least for a while, Spanish did have the "sh" sound and wrote it with the letter x. Specifically, I was under the impression that this is why "Don Quixote" becomes "Don Quichotte" in French. Can someone on MO either confirm this or assure me that it's nonsense? $\endgroup$
    – Andreas Blass
    Commented Aug 5, 2012 at 21:30
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    $\begingroup$ Well, pirate's maps used X's to denote the location of the treasure... :-) $\endgroup$
    – Mariano Suárez-Álvarez
    Commented Aug 5, 2012 at 22:49
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    $\begingroup$ Similar question: mathoverflow.net/questions/30307/… $\endgroup$
    – Alexander Chervov
    Commented Dec 7, 2012 at 18:53

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Interesting. Cajori says: "Nor is there historical evidence to support the statement found in Noah Webster's Dictionary, under the letter x, to the effect that 'x was used as an abbreviation of Ar. shei (a thing), something, which, in the Middle Ages, was used to designate the unknown, and was then prevailingly transcribed as xei.'"

The Oxford English Dictionary agrees with Cajori.

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    $\begingroup$ The passage from Cajori's A History of Mathematical Notations appears near the end of item #340 on pages 382-383 of the Dover edition. The entire entry, on Descartes' z, y, x, can be found online by googling on, say "Nor is there historical evidence to support the statement found in Noah" and clicking through to the googlebooks page. $\endgroup$
    – Barry Cipra
    Commented Aug 5, 2012 at 20:54
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“Finally, from so little sleeping and so much reading, his brain dried up and he went completely out of his mind.” ― Miguel de Cervantes Saavedra, Don Quijote

The name of "Don Quijote" on the original version of Cervantes was spelt "Don Quixote", but the pronunciation was like the Spanish "j". There is no "sh" sound in modern Spanish, and according to some experts, there was none at least to Cervantes times. So the explanation about the "X" sounds plausible.

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    $\begingroup$ According to en.wikipedia.org/wiki/Old_Spanish_language the "x" was pronounced as "sh" until the early 15th century, i.e., the tail end of Arabic occupation. This makes the proposed explanation less plausible. $\endgroup$
    – S. Carnahan ♦
    Commented Aug 6, 2012 at 0:42
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    $\begingroup$ “X” is also pronounced as “sh” in modern Iberian languages other than Castilian, such as Catalan, Leonese, Galician, and Portuguese. $\endgroup$
    – Emil Jeřábek
    Commented Aug 6, 2012 at 10:26
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    $\begingroup$ Exactly! The "sh" sound exists, as you say, in some of the languages of the Iberian peninsula. It also existed in medieval Spanish, but there was a change in the phonetics shortly after the Arabs left, and this change had already been accepted to Cervantes' times. The way of writing the new sound "j" was introduced as a norm in the XVII century. $\endgroup$
    – Carmen Rovi
    Commented Aug 6, 2012 at 11:38

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