Technically it is a grapheme developed from a digraph.
Æ (minuscule: æ) is a grapheme formed from the letters a and e. Originally a ligature representing a Latin diphthong, it has been promoted to the full status of a letter in the alphabets of many languages. As a letter of the Old English alphabet, it was called æsc ‘ash tree' after the Anglo-Saxon futhorc rune ᚫ which it transliterated; its traditional name in English is still ash(IPA: /æʃ/).
In typography, a grapheme is the fundamental unit in written language. Graphemes include alphabetic letters, Chinese characters, numerals, punctuation marks, and all the individual symbols of any of the world's writing systems.
In a phonemic orthography, a grapheme corresponds to one phoneme. In spelling systems that are non-phonemic – such as the spellings used most widely for written English – multiple graphemes may represent a single phoneme. These are called digraphs (two graphemes for a single phoneme) and trigraphs (three graphemes). For example, the word ship contains four graphemes (s, h, i, and p) but only three phonemes, because sh is a digraph.
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