The baseline is like an imaginary invisible line upon which letters appear to sit.
A typeface’s x-height, as its name suggests, is the height of the lowercase x measured from the baseline. The ascenders of letters reach above the x-height while descenders hang below the baseline.
Even at the same size, the x-height between different typefaces can differ. In this image, each letter x is the same point size, but notice the differences in x-height:
This article is the first instalment of the Anatomy of Type series. To learn more about more about typography, design principals, and colour check out my Pluralsight Introduction to Design course.