In typography, x-height is the distance between the baseline of a line of type and tops of the main body of lowercase letters (i.e. excluding ascenders or descenders). The x-height is a factor in typeface identification and readability.
Typefaces with very large x-height relative to the total height of the font have shorter ascenders and descenders and thus less white space between lines of type. Sans serif typefaces typically have large x-heights.
Issues With X-Height
In typefaces with small x-heights, other letter parts such as ascenders and descenders may become more visually noticeable.
Typefaces with large x-heights may appear darker, heavier, crowded, and more difficult to read at body copy sizes.