A ligature is the combination of two or more characters to create a single character or syllable. For example, in English, a diphthong is a ligature which unites two vowel characters to create another sound: a + e = æ.
In Arabic, ligatures are the combinations of two and, sometimes, three characters into one glyph. The resulting glyph replaces the characters composing it. For example, an Arabic letter typed twice is stored in memory as two distinct keystrokes, occupying two display cells. However, the Arabic language engine recognizes the context of each character and returns one glyph as shown in Figure 1-3.
In the Arabic language engine, a ligature occupies the same number of display cells as input characters, with one exception: the combination character lamalif.
The rules governing ligatures in Arabic text are very complex and do not depend solely on individual characters. Certain fonts define as many as 200 ligatures, while other fonts do not use ligatures at all.