In typography, kerning (less commonly mortising) is the process of adjusting the spacing between characters in a proportional font, usually to achieve a visually pleasing result. Kerning is the adjustment of the space between individual letter forms vs. tracking which is the uniform adjustment of spacing applied over a range of characters.
In a well-kerned font, the two-dimensional blank spaces between each pair of characters all have similar area. The related term kern denotes a part of a type letter that overhangs the edge of the type block.
The word kern is 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 letter-piece. Such notched pieces were only set against one another, not against unnotched ones, which had straight sides. The corner allowed for a character's features to reach into the area normally taken up by the next character, for example the top bar of the T, or the right diagonal stroke of the V to hang over the bottom left corner of an A.
Having a consistently shaped corner cut out allowed for using fewer pieces of type to make up all possible kerning pairs; for example a T- and V-piece with kerning on the right would match the same A piece with a matching kerning indention on the left.
In digital typography we can adjust the "kern" of letters or as in most cases let the program do it for us. Programs seldom a great job of kerning without help from a special section of the program and the operator.
In digital typography, kerning is usually applied to kerning pairs as a number to be added to the default character spacing, expressed in the font's coordinate system. A digital font's kerning feature can also increase the character spacing between two characters. Increased character width is used mainly in conjunction with accented letters.
Another approach is to use kerning classes; where one offset is stored for any pair of characters from two sets, for example (V, W) and (a, e, o). This one class is equivalent to the pairs Va, Wa, Ve, We, etc. Kerning classes can be used in OpenType fonts, and applications that support this feature. Although this is the newest, most advanced form of kerning, using kerning classes is essentially the same approach as was used in metal type. The use of kerning classes is necessitated mostly by today's multi-language fonts that feature many more glyphs, and more kerning pairs, than a single language font would need; especially accented letters.
Kerning is also widely used 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 lower case letters on the right side, such as the combination Ro. It is also used to fit a period (full stop) closer to these and to F, as well as the lower case letters y and r. Some other combinations are AC, FA, and OA.
Which letters need to be kerned depends on the languages the font is to be used with. Some combinations of letters are not used in normal words in any language, so to include kerning for these combinations is not necessary.
Kerning is implicitly part of digital type design, and advanced typographic systems allow the specification of kerning. It is commonly confused with tracking. Most high-quality fonts contain instructions for kerning which are applied automatically by the typesetting engine.
Non-proportional (monospaced) fonts don't use kerning, since their characters by definition always have the same spacing.