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A trigraph (from the Greek: τρεῖς, treîs, "three" and γράφω, gráphō, "write") is a group of three letters used to represent a single sound or a combination of sounds that does not correspond to the written letters combined.
For example, in the word schilling, the trigraph sch represents the voiceless postalveolar fricative /ʃ/, rather than the consonant cluster */skh/. In the word beautiful, the sequence eau is pronounced /juː/, and in the French word château it is pronounced /o/. It is sometimes difficult to determine whether a sequence of letters in English is a trigraph, because of the complicating role of silent letters. There are few productive trigraphs in English; one is tch as in watch.
The trigraph sch is German, where it is equivalent to the English sh; like English sh, it is not regarded as an independent letter of the alphabet. In Hungarian, the trigraph dzs is treated as a distinct letter, with its own place in the alphabet. It is pronounced like an English "j" /dʒ/. The combination gli in Italian can also be a trigraph, representing the palatal lateral approximant /ʎ/ before vowels other than i.
Trigraphs in non-Latin scripts
Although trigraphs are not uncommon in Latin alphabets, they are rare elsewhere. There are several in Cyrillic alphabets, which for example uses five trigraphs and a tetragraph in the Kabardian alphabet: гъу /ʁʷ/, кӀу /kʷʼ/, къу /qʷʼ/, кхъ /q/, хъу /χʷ/, and кхъу /qʷ/. While most of these can be thought of as consonant + /w/, the letters in кхъ /q/ cannot be so separated: the х has the negative meaning that кхъ is not ejective, as къ is /qʼ/. (See List of Cyrillic digraphs.)
Hangul has a few vowel trigraphs, ㅙ /wɛ/ and ㅞ /we/ (from oai and uei), which are not entirely predictable. There is also a single obsolete consonant trigraph, ㅹ *[v̥], a theoretical form not actually found in any texts, which is the digraph ㅃ *[b̥] plus a bottom circle used to derive the labio-dental series of consonants.
American Sign Language uses a multigraph of the American manual alphabet to sign 'I love you', from the English initialism ILY. It consists of the little finger of the letter I plus the thumb and forefinger of the letter L. It is conceived of as a trigraph of the letters I-L-Y, though the letter Y (little finger and thumb) overlaps with the other two letters.
Japanese kana use trigraphs for Cyō sequences, as in きょう kyou /kjoo/ 'today'; the う is only pronounced /o/ after another /o/.