Kerning

Why is kerning necessary
The type designer can specify a fixed amount of white space on either side of a character, but it is always a compromise because of the many different shapes of characters. While many character combinations can be handled properly by skilful setting of the default spacing, a fair number of combinations need to be handled by kerning. Also, uppercase characters are usually spaced so they fit best when followed by a lowercase letter, since this is how they are mostly used. That means that words set in all uppercase characters are not spaced well, and therefore require kerning.

How kerning data is created
Most commercial font development tools provide the ability to display characters on the screen, in combination with other characters, where the designer can make judgements as to what the kerning adjustments should be. Screen resolution is too coarse for final decisions, however, and the combinations should be proofed on a laser printer, or better yet, on a high resolution imagesetter.

Where kerning data is stored
In Windows, the kerning data for a Type 1 font is stored in the PFM (Printer Font Metrics) file installed in the user's system. On the Macintosh, the data is stored in the Suitcase file which is installed for each Type 1 font family. In a UNIX system, the kerning data is stored in the AFM (Adobe Font Metrics) file associated with each font.

Kerning Should be a User Option
The automatic use of kerning in an application should always be an option for the user, since there can be a significant performance slow-down when kerning is used. Every time text is edited, hundreds if not thousands of data items must be searched and applied to the appropriate line-breaks. Since many users do not require kerning, while other users may insist on it, it should always be a user-determined feature.

Number of Kerning Pairs
There is no consensus on the ideal number of kerning pairs that should be specified in a font; the number ranges from maybe 30 to over 4000. Adobe uses approximately 280 in standard Type 1 fonts, and approximately 800 in an Adobe Originals font. Some font developers provide thousands of kern pairs - but quantity is no guarantee of quality. Sometimes a font's default spacing is not done correctly, and the designer tries to compensate by supplying an excessive amount of kerning. Kerning should only be attempted after the font has been completely and precisely fitted. It is possible that fonts intended for display use for the advertising design market may require more kerning pairs than a text font because of the tendancy to use them at a large size and very tightly spaced.

Support by Software Applications
Not all applications support pair kerning - such as most word processors. If a customer asks why the kerning looks so bad when using such an application, it may be because the application doesn't support it! Also, Microsoft Word(R), for example, supports kerning for TrueType but not for Type 1 fonts.

User Customization of Kerning Data
Users can customize a font's kerning with a kerning editor utility. Problems can arise when one user customizes a font's kerning, and then sends a document to another user (or service bureau) who has the font as it came from the original vendor. However, if a font with custom kerning is embedded in an Acrobat PDF document, the custom kerning will be preserved on the viewer's system.

Sample List of Character Pairs
An example of a list of kerning pairs and the associated data, as taken from a typical Roman Type 1 text font, can be seen in Pair Kerning Data Example.