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.
Latin-script trigraphs[edit]
Further information: List of Latin-script trigraphs
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[edit]

ASL short for "I love you"
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...