In typography

Typography

Typography is the art and technique of arranging type in order to make language visible. The arrangement of type involves the selection of typefaces, point size, line length, leading , adjusting the spaces between groups of letters and adjusting the space between pairs of letters...

, the mean line, also (and more simply) known as midline, is the line that determines where non-ascending lowercase letters

Letter (alphabet)

A letter is a grapheme in an alphabetic system of writing, such as the Greek alphabet and its descendants. Letters compose phonemes and each phoneme represents a phone in the spoken form of the language....

 terminate in a typeface

Typeface

In typography, a typeface is the artistic representation or interpretation of characters; it is the way the type looks. Each type is designed and there are thousands of different typefaces in existence, with new ones being developed constantly....

. The distance between the baseline

Baseline (typography)

In European and West Asian typography and penmanship, the baseline is the line upon which most letters "sit" and below which descenders extend.In the example to the right, the letter 'p' has a descender; the other letters sit on the baseline....

and the mean line is called the x-height

X-height

In typography, the x-height or corpus size refers to the distance between the baseline and the mean line in a typeface. Typically, this is the height of the letter x in the font , as well as the u, v, w, and z...

.

Round glyphs will break (overshoot

Overshoot (typography)

In typeface design, the overshoot of a round or pointed letter is the degree to which it extends higher or lower than a comparably sized "flat" letter , to achieve an optical effect of being the same size....

) the mean line slightly in many typefaces, since this is aesthetically more pleasing; a rounded shape will appear visually smaller than flat topped (or bottomed) shapes of equal height, due to an optical illusion.
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