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 adjusts the space between individual letter forms, while tracking (letter-spacing) adjusts spacing uniformly 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.
In metal typesetting 
The source of the word kern is from the French word carne, meaning "projecting angle, quill of a pen". The French term originated from the latin Latin cardo, cardinis, meaning "hinge". 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 a T or the right diagonal stroke of the V could 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.
Digital typography 
In metal typesetting, kerning was labour-intensive and expensive because the matrices had to be physically modified. It was therefore only employed on letter combinations which needed it the most, such as VA or AV. With the arrival of digital fonts, it became much easier to kern many glyph combinations.
In digital typography, kerning is usually applied to letter pairs as a number by which the default character spacing should be increased or decreased. Reducing the default character spacing is 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 that often use kerning are AC, FA, and OA. Increased character width is used mainly in conjunction with accented letters.
Which letters need to be kerned depends on with which languages the font is to be used. Since some combinations of letters are not used in normal words in any language, kerning these is not necessary. Non-proportional (monospaced) fonts do not use kerning, since their characters always have the same spacing.
Kerning pairs 
The OpenType format permits group-based kerning, which facilitates kerning for fonts that have a large number of glyphs. Instead of specifying the kerning for ‘Va’ and ‘Vá' separately, all diacritics with the base letter 'a' are placed into a group and the kerning between V and this group is specified. At the same time it is possible to add exceptions, e.g. for ‘Vā’, to the group. Group-based kerning is supported in nearly all modern office and desktop publishing applications.
Some typographic programs provide an autokerning feature. Autokerning simply takes into account a predefined list of common kerning pairs, and if the outlines of two consecutive glyphs are spaced too far apart, makes a kerning entry. Autokerning is especially useful for kerning multi-language fonts. However, it is rarely a sufficient alternative for manual kerning, as some characters may appear to an algorithmic comparison to be spaced very closely together, but to a human reader might appear to be spaced too far apart, especially when the only part of a glyph that is 'too close' is a diacritic sign.
Kerning tools 
Some page layout programs allow the user to kern characters within their text. However, one must use a font editor to permanently change the kerning of a font.
Kerning in Browsers 
The CSS property
text-rendering: optimizeLegibility; enables kerning in Firefox, Chrome and Safari. There is also the proposed CSS3 property
font-kerning, but it is only supported in Firefox, Chrome and Internet Explorer 10. The CSS3 draft states that kerning should only be enabled for OpenType fonts.
See also 
- "Fonts : Type topics: Glossary". Adobe. Retrieved 2011-09-16.
- "kern". Dictionary.com. Retrieved 13 November 2012.
- The font-kerning property in the 2012's CSS3 Draft: http://www.w3.org/TR/css3-fonts/#font-kerning-prop
The dictionary definition of kerning at Wiktionary