Have you ever wondered on your math class, you always see and use the X variable as unknown? Do you know where it came from or at least who was the responsible for it?
The answer is because a famous Frenchman decided so. He also decided not for the sake of aesthetics — he just ran out of other letters in the printing press.
His name is Rene Descartes and perhaps the most famous mathematician in the world. The most known contribution of him is the Cartesian coordinate system that we study at school and also the famous saying “Cogito ergo sum” (I think, therefore I am) belongs to him.
Descartes was greatly appreciated by Cardinal Richelieu and helped him to publish a series of famous books in France. At that time, there was no single decision on what letters to denote unknowns. Some mathematicians used vowels and A, B and C were also used.
However, Descartes took a different path. In 1637, he published his philosophical work, Discourse on Method. But it was not this book that brought him fame, but the appendix to it, La Geometrie. In it, the mathematician uses X, Y and Z to denote unknown quantities, and he used the letters A, B and C to denote the coefficients. The reason why he did this is purely practical. Typographers at the time had a lot of extra X, Y and Z letters.
At that time, books were not printed by machines. These were people who took letters out of boxes and put them in special cells. The letters were covered with paints and books were published at a fantastic speed for that time. Up to 100 prints of one page could be made in an hour.
Several books could be printed at the same time. There were not enough common letters. But the letters X, Y and Z are rare in French and the printers advised Descartes to choose them.
This gave him a competitive edge. His books began to be published faster than those of other mathematicians, who often had to wait their turn.
La Geometrie turned out to be a very important book for the development of mathematics. He brought together geometry, algebra and made a great contribution to the formation of mathematical analysis. In geometry, Descartes proposed defining curves using equations. This is how graphs and functions appeared in our mathematics but X, Y and Z become the standard.
Interestingly, this had an impact not only on science and mathematic but on the entire human culture. Now X is associated with something mysterious or unknown. Think of the X-files and the X-Men.
Does (e)X in relationship counts as unknown as well since we should never look it again?
What does the equation means?
 Melissa. (2014, November 11). Why We Use "X" as the Unknown in Math. Retrieved from https://gizmodo.com/why-we-use-x-as-the-unknown-in-math-1657254357
 Florian Cajori, A History of Mathematical Notations (1929)