A trigraph (from the Greek: , treîs, "three" and , gráph?, "write") is a group of three characters 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 /?/, rather than the consonant cluster /sx/. In the word beautiful, the sequence eau is pronounced /ju:/, 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 however a few productive trigraphs in English such as tch as in watch, and igh as in high.
The trigraph sch in German is equivalent to the English sh and pronounced /?/. In the Dutch language, which is closely related to German, this same trigraph is pronounced /sx/. In Italian, however, sch represents the sounds /sk/ before e or i, as in "bruschetta" /brus'k?t:a/. In neither language is this trigraph 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" /d?/. The combination gli in Italian can also be a trigraph, representing the palatal lateral approximant /?/ before vowels other than i, as in aglio, pronounced [/'a?:o/].
Although trigraphs are not uncommon in Latin-script alphabets, they are rare elsewhere. There are several in Cyrillic alphabets, which for example uses five trigraphs and a tetragraph in the Kabardian alphabet: //, /k?'/, /q?'/, /q/, and //, and also a tetragraph ? /q?/. While most of these can be thought of as consonant + /w/, the letters in /q/ cannot be so separated: the ? has the negative meaning that is not ejective, as is /q'/. (See List of Cyrillic digraphs.)
Tsakonian has /t?/.
Hangul has a few vowel trigraphs, ? /w?/ and ? /we/ (from oai and uei), which are not entirely predictable. However, as ?/?/ and ?/e/ are considered as single letters in modern Korean, ? and ? are considered as digraphs now. There is also a single obsolete consonant trigraph, ?[?], a theoretical form not actually found in any texts. It is composed of digraph ?[b] and a circle-shaped single letter ?, which means the letter "to lighten" sounds, linguistically to change stop consonants to the fricative consonants in cases of bilabial consonants (for ?, ? changes alveolar tap to alveolar lateral approximant or retroflex lateral approximant). Because these letters are created to transcribe consonants of Mandarin (late imperial lingua franca), these are disappeared soon. In modern days, ? is used for different sound, [p?].
Japanese kana use trigraphs for (C)y? sequences, as in kyou /kjoo/ ("today"); the ? is only pronounced /o/ after another /o/.
In Inuktitut syllabics, the digraph ? ng cannot be followed by a vowel. For that, it must form a trigraph with g:
It also forms a trigraph with n for .
The sequence of letters making up a phoneme are not always adjacent. This is the case with English silent e, which has been claimed to modify preceding digraphs as well as preceding single vowel letters. For example, the sequence ou...e has the sound /u:/ in English joule. There are twelve such combinations in English, ⟨ai--e, au--e, ea--e, ee--e, ei--e, eu--e, ia--e, ie--e, oi--e, oo--e, ou--e, ui--e⟩, though it's been argued that a trigraph analysis is unnecessary.
The Indic alphabets are distinctive for their discontinuous vowels, such as Thai ?...? /?/, ?... /?/, ?... //. Technically, however, these may be considered diacritics, not full letters; whether they are trigraphs is thus a matter of definition, though they can in turn take modifying vowel diacritics, as in /ia?/ and /?a?/.