The predominant use of the letter x to represent an unknown value came about in an interesting way. During the printing of La geometrie [by Descartes] … the printer reached a dilemma. While the text was being typeset, the printer began to run short of the last letters of the alphabet. He asked Descartes if it mattered whether x, y, or z was used in each of the book’s many equations. Descartes replied that it made no difference which of the three letters was used to designate an unknown quantity. The printer selected x for most of the unknowns, since the letters y and z are used in the French language more frequently than is x
- from Unknown Quantity, pg 93 (which quotes from Art Johnson’s Classic Math)
That is, a printer set the convention on how we represent variables in modern mathematics based on the frequency of letter occurrence in the French language.