Trigraphen

Trigraphen are sequences of three letters that function as a single unit in a writing system. They are distinguished from digraphs (two-letter combinations) and tetragraphs (four-letter sequences) by length and their role in representing a single sound or spelling value. Trigraphen occur in various languages and can encode different phonetic or orthographic outcomes depending on the language.

In natural languages, trigraphen map to a single phoneme or serve as a distinct spelling unit within a word. Examples include the English tri-letter sequence "igh" as in sight, which represents the diphthong /aɪ/; and "tch" as in watch, which represents /tʃ/. German uses "dsch" to denote /dʒ/ in words such as Dschungel, while French employs "eau" to represent the vowel sound /o/. Trigraphen can also be context-dependent, occasionally signaling more complex pronunciations or orthographic distinctions in loanwords and dialectal spellings.

In computing, the term Trigraphen refers to a different concept within the C family of languages. C trigraphs are sequences of three ASCII characters beginning with two question marks, such as ??= for the character '#', ??/ for '\\', or ??' for '^'. They are processed by the compiler to yield the corresponding single character, a feature intended to improve portability on systems lacking certain symbols. Trigraphs in this sense are regarded as largely obsolete in modern programming practice but remain part of the language standard.

Overall, Trigraphen describe multi-letter sequences with specialized functional roles in both linguistic orthography and legacy programming, illustrating how three-letter units can encode a range of sounds or symbols.