In typography, two or more characters designed as a distinct unit and commonly available as a single character. There are five f-ligatures (fi, fl, etc.) plus the diphthongs (Æ, Œ, etc.). Gutenberg's font had many ligatures in order to simulate handwriting and to achieve even word spacing in his justified text columns.
The traditional ligatures are easily and automatically generated on command. Although book production most often finds them mandatory, advertising typography rarely finds them useful—and, in fact, they cannot be used in copy set tighter than normal spacing, since the space within ligatures cannot be manipulated. Computer automation will usually allow any ligature to be selected without operator intervention—an incentive for the expansion of ligature design and use until we someday develop a modern version of the Gutenberg font, which, research suggests, may increase legibility.
The diphthongs are also considered ligatures. Historically, ligatures were developed for metal type so that certain letter combinations that contained space buried between them could be used more closely together. Different languages had additional ligatures for often-used letter combinations. In German, for example, "ch" and "ck" were ligatures. The word ligature itself comes from the Latin word ligatus, the past participle of the word ligare, meaning "to tie or bind." Ligature also refers to any medical procedure in which arteries or tubes are "ligated" or tied, as in tubal ligation.