A tetragraph (from the Greek, Modern (1453-);: 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.[1] 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 pronounced as //q͡χʷ//. The Romanized Popular Alphabet created for the Hmong languages includes three tetragraphs: nplh, ntsh, and ntxh, which represent complex consonants.
See also: List of Latin-script tetragraphs.
In Cyrillic used for languages of the Caucasus, there are tetragraphs as doubled digraphs used for 'strong' consonants (typically transcribed in the IPA as geminate), and also labialized homologues of trigraphs.
(кхъу) is used in Kabardian for pronounced as /[qʷ]/, the labialized homologue of (кхъ) pronounced as /[q]/, in turn unpredictably derived from ejective (къ) pronounced as /[qʼ]/.
(кӏкӏ) is used in Avar for pronounced as /[kʼː]/, the 'strong' homologue of (кӏ) pronounced as /[kʼ]/, the ejective ((ӏ)) homologue of (к) pronounced as /[k]/. It is often substituted with (кӏ) pronounced as /[kʼ]/.
(цӏцӏ) is used in Avar for pronounced as /[tsʼː]/. It is often substituted with (цӏ) pronounced as /[tsʼ]/.
(чӏчӏ) is used in Avar for pronounced as /[tʃʼː]/. It is often substituted with (чӏ) pronounced as /[tʃʼ]/.
(гъӏв) is used in Archi for pronounced as /[ʁʷˤ]/
(ккъӏ) is used in Archi for pronounced as /[qːʼˤ]/
(къIв) is used in Archi for pronounced as /[qʼʷˤ]/
(ллъв) is used in Archi for pronounced as /[ɬːʷ]/
(ххьI) is used in Archi for pronounced as /[χːˤ]/
(хъIв) is used in Archi for pronounced as /[qʷˤ ]/
(хьIв) is used in Archi for pronounced as /[χʷˤ]/
Inuktitut syllabics has a series of trigraphs for ŋ followed by a vowel. For geminate ŋŋ, these are form tetragraphs with n:
ᙱ ŋŋi, ᙳ ŋŋu, ᙵ ŋŋaThese are literally nnggi, nnggu, nngga.