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Ligature (typography)

In writing and typography, a ligature occurs where two or more letterforms are written or printed as a unit. Generally, ligatures replace characters that occur next to each other when they share common components. Ligatures are a subset of a more general class of figures called "contextual forms". Contextual forms describe the case where the particular shape of a letter depends on its context (surrounding letters, whether or not it's at the end of a line, etc.).

One of the most common ligatures is "fi." Since the dot above a lowercase "I" interferes with the loop on the lowercase "F," when "f" and "i" are printed next to each other, they are combined into a single figure with the dot absorbed into the "f," which appears as "fi".

An example of a more general contextual form is the Greek lowercase sigma. When typesetting Greek, the selection of which sigma to use is determined by whether or not the letter occurs at the end of the word.

Ligatures were originally used by medieval scribes to conserve space and increase writing speed. A 14th century manuscript, for example, will include hundreds of ligatures. Early typefaces used ligatures in order to emulate the appearance of hand-lettered manuscripts. As typesetting became more automated, most of these ligatures fell out of common use. It is only recently that computer based typesetting has encouraged people to start using them again (although "fine art" printers have used them all along). Generally, ligatures work best in typefaces which are derived from calligraphic letterforms. Also useful are contextual forms, such as swash capitals, terminal characters, and so on.

A good example of a computer typeface with a moderately rich set of ligatures is the Computer Modern Roman typeface that is provided with TeX, which includes the ff, fi, fl, ffi, and ffl ligatures which TeX automatically uses when it finds these letters juxtaposed in the text.

This table shows unligatured sets of letters on the left, and the corresponding Unicode ligature on the right. Not all browser/operating system combinations will render the table correctly.
AEÆ
aeæ
OEŒ
oeœ
ffff
fifi
flfl
ffiffi
fflffl
ijij
st longſt
st smallst
szß

It is important to note that the letter Æ (æ) when used in the Danish or Norwegian languages is not a typographical ligature, and must never be treated as such. It is a distinct letter and vowel. Likewise, the letter ß is a distinct letter in the German, and the letter ij/IJ is a distinct letter in Dutch.

See also: Danish, Norwegian and German alphabet.

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