A trigraph (from the Greek words tria = three and grapho = write) is a group of three letters used to represent a single sound. For example, in the word schilling, "sch" represents the 'sh' sound. In the word "night", "igh" represents the vowel (a diphthong). Other examples are "beautiful" (eau), "adieu" (ieu).
Some languages use trigraphs to represent their native triphthongs when using plain Latin alphabet without diacritics.
See digraph for more explanations.
Longer "multigraphs" are also known. It is quite possible that the longest one is an "octagraph" schtschj used in German language to represent a Russian languagepalatalizedphoneme щь (which is, by the way, represented by a digraph in Russian).
In a phonological 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 phonemes.
These are called digraphs (two graphemes for a single phoneme) and trigraphs (three graphemes).
ORTHOGRAPHY Concise Oxford Companion to the English Language...
The failure of a proof-reader to notice gross errors in the meaning of the text being checked, because different and incompatible levels of processing are required for checking meaning and orthography.
Whereas, in standard orthography, the same letters can be used to represent different sounds (the y in sky and syrup), and different combinations of letters can be used to represent the same sound (the...