Kerning

In typography, kerning 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 a visually similar area.

The related term kern denotes a part of a type letter that overhangs the edge of the type block.

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 cardo, cardinis, meaning "hinge". In the days when all type was cast metal, parts of a sort that needed to overlap adjacent letters simply hung off the edge of the sort slug. The bit of metal that hung over the edge was called a kern. At that time, the word "kerning" only referred to manufacturing the sorts with kerns, while adjusting space between letters during compositing was called inter-spacing or letter spacing.

Because this method was not well-suited to some pairs of letters, ligatures were supplied for those glyph combinations, such as the French L’, or the combinations ff, fi and ffi.

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: a positive value for an increase, a negative value for a decrease. The number is expressed in font units, one unit being a certain fraction of an em (one em is the type size currently used). Different fonts may use different units, but common values are 1000 and 2048 units/em. Thus, for 1000 units/em, a kerning value of 15 means an increase in character spacing by 0.015 of the current type size. (The kerning units for a given font are the same as the units used to express the character widths in that font.)

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