A tetragraph (from the Greek: -, tetra-, "four" and , gráph?, "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 ? for /q/. The Romanized Popular Alphabet created for the Hmong languages includes three tetragraphs: nplh, ntsh, and ntxh, which represent complex consonants.
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.
⟨?⟩ is used in Kabardian for [q?], the labialized homologue of ⟨⟩ [q], in turn unpredictably derived from ejective ⟨⟩ [q'].
⟨?⟩ is used in Avar for [k':], the 'strong' homologue of ⟨⟩ [k'], the ejective (⟨?⟩) homologue of ⟨?⟩ [k]. It is often substituted with ⟨⟩ [k'].
⟨?⟩ is used in Avar for [ts':]. It is often substituted with ⟨⟩ [ts'].
⟨?⟩ is used in Avar for [t?':]. It is often substituted with ⟨⟩ [t?'].
⟨?⟩ is used in Archi for 
⟨?⟩ is used in Archi for [q:'?]
⟨I?⟩ is used in Archi for [q']
⟨?⟩ is used in Archi for [?:?]
⟨I⟩ is used in Archi for [?:?]
⟨I?⟩ is used in Archi for [q ]
⟨I?⟩ is used in Archi for 
Inuktitut syllabics has a series of trigraphs for ? followed by a vowel. For geminate , these are form tetragraphs with n:
These are literally nnggi, nnggu, nngga.