X-Height
The height of lowercase letters such as x, a, and e, measured from the baseline to the top of the main body (excluding ascenders and descenders).
Typefaces with larger x-heights appear more legible at small sizes because the main body of each letter is bigger relative to the em square. X-height is a key factor when pairing heading and body fonts.
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Anatomy of TypeRelated terms
Baseline
The invisible line upon which most letters sit; the reference line for vertical alignment and rhythm in typography.
Cap Height
The height of capital letters measured from the baseline to the top of flat capitals like H or E.
Ascender
The part of a lowercase letter that extends above the x-height, as in b, d, h, and l.
Descender
The part of a lowercase letter that extends below the baseline, as in g, j, p, q, and y.