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 http://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.)