Tetragraph

A tetragraph (from the Greek: τετρα-, tetra-, "four" and γράφω, gráphō, "write") is a sequence of four letters used to represent a single sound (phoneme), or a combination of sounds, that do not necessarily correspond to the individual values of the letters. In German, for example, the tetragraph tsch represents the sound of the English digraph ch. English does not have tetragraphs in native words (the closest is perhaps the sequence -ough in words like through), but chth is a true tetragraph when found initially in words of Greek origin such as chthonian.

Phonemes spelled with multiple characters often indicate that either the phoneme or the script is alien to the language. For example, the Cyrillic alphabets adapted to the Caucasian languages, which are phonologically very different from Russian, make extensive use of digraphs, trigraphs, and even a tetragraph in Kabardian кхъу for /qʷ/. The Romanized Popular Alphabet created for the Hmong languages includes three tetragraphs: nplh, ntsh, and ntxh, which represent complex consonants.