At the end of 16th century François Viète introduced the idea of representing known and unknown numbers by letters, nowadays called variables, and of computing with them as if they were numbers, in order to obtain the result by a simple replacement. Viète's convention was to use consonants for known values and vowels for unknowns.
In 17th century, René Descartes began representing unknowns in equations by x, y, and z, and knowns by a, b, and c.
Also in the 17th century, Pierre de Fermat wrote in the margin of a book that the equation an + bn = cn had no solutions in positive integers, if n is an integer greater than 2. This, obviously, is a blending of the two conventions of Viète and Descartes.