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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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closed as off topic by Franz Lemmermeyer, Felipe Voloch, Suvrit, Simon Thomas, Asaf Karagila Aug 5 '12 at 22:44

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According to, Florian Cajori's History of Mathematical Notations says there is no evidence for this theory. – Henry Cohn Aug 5 '12 at 18:16
@Henry that should be an answer not comment. – Alexander Chervov Aug 5 '12 at 18:45
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? – Andreas Blass Aug 5 '12 at 21:30
Well, pirate's maps used X's to denote the location of the treasure... :-) – Mariano Suárez-Alvarez Aug 5 '12 at 22:49
@Mariano I should probably ask next MO question if any pirate-mathematician known in history or at least pirate influenting some mathematicians? :-) – Alexander Chervov Aug 6 '12 at 10:49
up vote 4 down vote accepted

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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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. – Barry Cipra Aug 5 '12 at 20:54

“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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According to 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. – S. Carnahan Aug 6 '12 at 0:42
“X” is also pronounced as “sh” in modern Iberian languages other than Castilian, such as Catalan, Leonese, Galician, and Portuguese. – Emil Jeřábek Aug 6 '12 at 10:26
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. – Carmen Rovi Aug 6 '12 at 11:38

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