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I can understand how the Greek alphabet came to be prominent in mathematics as the Greeks had a huge influence in the math of today. Certain letters came to have certain implications about their meaning (i.e. $\theta$ is almost always an angle, never a function).

But why did $x$ and $y$ come to prominence? They seem like $2$ arbitrary letters for input and output, and I can't think why we began to use them instead of $a$ and $b$. Why did they become the de facto standard for Cartesian coordinates?

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    $\begingroup$ You can also see this MO question: mathoverflow.net/questions/30307/… $\endgroup$
    – damiano
    Aug 21, 2010 at 16:30
  • $\begingroup$ Sorry, didn't see you'd already linked to it when I posted my answer. Hope you don't mind me leaving my response up though, as it does answer the question. $\endgroup$ Aug 21, 2010 at 21:52
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    $\begingroup$ So you have never heard of theta functions? $\endgroup$
    – GEdgar
    Aug 21, 2010 at 23:09
  • $\begingroup$ GEdgar: On the other hand, I more often see $\vartheta$ for theta functions, and $\theta$ for angle-related things. $\endgroup$ Aug 21, 2010 at 23:50
  • $\begingroup$ Probably because $a$ already has a role in English (e.g. a dog, a cat), so its just less confusing to use $x$ and $y$. $\endgroup$ Nov 4, 2013 at 3:19

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This question has been asked previously on MathOverflow, and answered (by Mariano Suárez-Alvarez). You can follow this link, and I quote his response below.

You'll find details on this point (and precise references) in Cajori's History of mathematical notations, ¶340. He credits Descartes in his La Géometrie for the introduction of x, y and z (and more generally, usefully and interestingly, for the use of the first letters of the alphabet for known quantities and the last letters for the unknown quantities) He notes that Descartes used the notation considerably earlier: the book was published in 1637, yet in 1629 he was already using x as an unknown (although in the same place y is a known quantity...); also, he used the notation in manuscripts dated earlier than the book by years.

It is very, very interesting to read through the description Cajori makes of the many, many other alternatives to the notation of quantities, and as one proceeds along the almost 1000 pages of the two volume book, one can very much appreciate how precious are the notations we so much take for granted!

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Classically "x" was always used in the sciences for denoting an unknown quantity, e.g. the "X-rays" of Röntgen.

Cajori has a nice discussion in "A History of Mathematical Notations". In brief, Descartes's convention was to use letters from the earlier half (a,b,c...) for known quantities and from the latter half (x,y,z...) for unknowns. This was in the 1600s if I remember correctly.

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    $\begingroup$ To future editors: either use "oe" or the umlaut; "Röentgen" just looks... ridiculous. Honestly, don't edit unless your grammar/phrasing is better than my formulation. :P $\endgroup$ Oct 6, 2015 at 23:17
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I read somewhere this convention was started by Rene Descartes. While conceptualizing the coordinate system, he used $x$ and $y$ to denote the axes. It took root from there on and has been used ever since.

I am sorry I can't remember the source now, but I will cite if I do remember later.

Descartes

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  • $\begingroup$ What a stunning picture. $\endgroup$
    – 000
    Jan 22, 2012 at 18:59
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I have no idea if this guy is right or not, but here's a different story.

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