Why is it called X height?
The X height of a typeface is the height of the lowercase letter, excluding the ascenders and descenders, i.e. the distance between the baseline and the tops of the letters. Ever wonder why is it called x height, and not any other letter?
When we analyse the anatomy of typography, we see that the curves of certain letters such as the ‘o’ or the ‘s’ may extend below the baseline, or above the x height line.
So we need a letter, that is flat at the top, and the bottom. moreover the serifs of the letter x are always horizontal, as opposed to the letter “z” which could have vertical serifs as well.
Finally, x is the only letter in the entire alphabet that is perfectly symmetrical both horizontally and vertically.
Therefore the letter ‘x’ can give the most accurate height of any typeface.