>Viète writes In artem analyticam isagoge (Introduction to the analytical art), using letters as symbols for quantities, both known and unknown. He uses vowels for the unknowns and consonants for known quantities. Descartes, later, introduces the use of letters x, y ... at the end of the alphabet for unknowns.
This may be my favorite point in the history of mathematics. Using placeholders for things (variables) and efficient notation makes reasoning easy. It's such an obvious thing now, but I think it's amazing.