In typography, kerning is the process of adjusting letter spacing in a proportional font. In a well-kerned font, the two-dimensional blank spaces between each pair of letters all have similar area. The name derives from a cognate of corner. In the days when all type was cast metal, a corner was notched to a consistent height on one or both sides of a piece of type. Such notched pieces were only set against one another, not against unnotched ones. An alternative was once to have ligatures for the common combinations made, such as the French ligature L'.
A simple proportional font will simply specify the right and left boundaries, called sidebearings, of each glyph. However, depending on the adjacent letter, the space may be reduced (and occasionally increased) to improve the overall appearance of the text. For example, A and V can be placed closer together so that the top left of the V is directly above the bottom right of the A.
Kerning is used primarily to fit capital letters, such as T, V, W, and Y, closer to some other capital letters on either side (especially A) and to some minuscule letters on the right side. It is also used to fit a period (full stop) closer to these and to F, as well as the minuscule letters y and r. There exist also the combinations AC, FA, and OA.
Kerning is implicitly part of digital type design, and advanced typographic systems allow the specification of kerning. It is commonly confused with tracking, but these are two separate concepts. Most high quality fonts contain instructions for kerning which are applied automatically by the typesetting engine.