Any letter can represent an unknown number, and the most commonly used ones are X, Y, Z, A, B and C. Equations with unknown numbers like this are called mathematical equations.

Graphics can also represent unknowns.

In Arabic, SHeenlan stands for something, while al-SHeenlan stands for something unknown.

But at that time, most Spaniards couldn't pronounce SH, so they used the "on" sound of CK in ancient Greece.

It is similar to Latin X in writing and gradually becomes X in common use.