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Does anybody know why traditionally has been the letter $x$ used as the incognita in an equation? And then following back: $y, w, \dots$

Why starting from the end of the alphabet?

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    $\begingroup$ Maybe it's because the first letters of the alphabet were already used for constants? $\endgroup$ – Bram28 Aug 15 '18 at 15:15
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    $\begingroup$ While questions about the history of mathematics are not off-topic here, I think that you are more likely to get a good answer if you ask the question on History of Science and Mathematics. While there is no clear consensus about what kinds of questions are more appropriate for that site, my reading of the discussion is that questions that are connected to other branches of science (as this one, I think, ultimately is) and questions that don't require deep mathematical knowledge (as this one does not) should, perhaps, be migrated. $\endgroup$ – Xander Henderson Aug 15 '18 at 16:36
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The convention of using letters at the front of the alphabet (a, b, c) for known cofficients and letters at the end of the alphabet (x, y, z) for unknowns originates with Descartes's La Géométrie from 1637.

I don't think it is known what inspired Descartes to exactly this convention, but the idea of using different groupings of letters for knowns and unknowns was already present with François Viète, whose works from the late 1500s were known to Descartes. Viète used consonants for constants and vowels for unknowns.

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Regarding $x$ there was a TED talk by Terry Moore: Why is 'x' the symbol for an unknown?, who claimed it came from the Arabic word al-shalan, which means "the unknown thing".

An article by Lauren Davis however sides rather with Descartes, citing historian Florian Cajori.

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