# Why is it that the alphabets X and Y always considered?

In school when we were asked to find the missing figure, we will find it by putting X as the missing value. If two values are to be found, then it is always X and Y. Even today, when we do not know the name of a person, we refer it to as Mr. X.Even in a graph, we have X-axis and Y-axis. Later when we studied chromosomes, we learned about X and Y chromosomes. Out of the 26 alphabets, why is it that only X and Y is given so much importance? I agree that A and B are also used at times like in formulas but not as much as X and Y.

Is there anything in particular with these alphabets? Have you ever had this doubt?