I heard or have read the following nice explanation for the origin of the convention that one uses (almost) always $x,y,z$ for variables. (This question was motivated by question
[Origin of symbol \*l\* for a prime different from a fixed prime?](https://mathoverflow.net/questions/30081/origin-of-symbol-l-for-a-prime-different-from-a-fixed-prime))


It seems this custom is due to the typesetter of Descartes. Descartes used initially other letters 
(mainly $a,b,c$) but the typesetter had the same limited number of lead symbols
for each of the 26 letters of the Roman alphabet. The frequent use of variables exhausted his stock and he asked thus Descartes if he could use the last three letters $x,y,z$ of the alphabet (which occur very rarely in French texts). 


Does anyone know if this is only a (beautiful) legend or if it contains some truth? (I checked that Descartes uses indeed already $x,y,z$ generically for variables in his printed works.)



Explanation why $x,y,z$ are always variables
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!


