Kerning
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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.
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Kerning in metal typesetting
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 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 lefthand top 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 indentation on the left.
An alternative was once to have ligatures for the common combinations made, such as the French ligature L'., or the common combinations ff, fi and ffi.
Example of kerning
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.
For an example, see the Figure on this page.
Kerning pairs
In digital typopgraphy, kerning is usually applied to kerning pairs as a number to be added of the default characterspacing, expressed in the font's coordinate system. For example, the kerning of VA in Adobe's Helvetica font is -80. A digital font's kerning feature can also increase the characterspacing between two characters; for example the kerning value for ry in Adobe's Helvetica is 30. Increased character width is used mainly in conjunction with accented letters.
Kerning classes
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 neccesitated mostly by today's multi-language fonts that feature many more glyphs, and more kerning pairs, than a single langauage font would need; especially accented letters.
Examples of kerned letters
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, 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 minuscule letters y and r. There exist also the combinations 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 aren't used in normal words in any language, so to include kerning for these combinations isn't necessary.
Autokerning
Some typographic programs provide an autokerning feature. Autokerning simply takes into account a predefined list of common kerning pairs and inspects the glyph outlines to see whether they are spaced too far apart. If they are, a kerning entry is made. Auto kerning is especially useful for kerning multi-language fonts. However, it's 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.
Uses of kerning
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.
Many word processors, such as Microsoft Word or OpenOffice.org don't enable kerning by default, resulting in less aesthetically pleasing output.
Non-proportional (monospaced) fonts don't use kerning, since their characters do not overlap, and consistent character spacing is needed to ensure characters always line out consistently.da:Knibning (typografi) de:Kerning pl:Kerning