X was especially hard to come up with a topic for. Hence why I have chosen a small (yet important) part of typography: x-height!
The x-height is the invisible line that most lowercase letters are as tall as. The exceptions to this are b, d, f, h, k, and l. For these letters, the x-height dictates the height of their waistline — the top of the body of the letter.
The reason the x-height is named thusly is because it marks the proper optical height of the lowercase letter x in a typeface. Sometimes, a type designer may make rounded letters — such as c, o, and e — slightly taller than the x-height to make up for any optical illusion that causes the letters to not look the same height.
Other important guidelines that dictate the height and length of letters are the ascender line, cap height, baseline, and the descender line.
I hope you’ve found this small glimpse into type helpful!