Hexagraph

A hexagraph (from the Greek: ἕξ, héx, "six" and γράφω, gráphō, "write") is a sequence of six letters used to represent a single sound (phoneme), or a combination of sounds that do not correspond to the individual values of the letters. They are found only in Irish orthography, and many of them can be analysed as a tetragraph followed by the vowels e or i on either side to indicate that the neighboruing consonants are palatalized ("slender"). However, not all Irish hexagraphs are analysable that way. The hexagraph oidhea, for example, represents the same sound (approximately the vowel in English "write") as the trigraph adh, and with the same effect on neighboring consonants.

English does not have hexagraphs. The six-letter sequence schsch appears in German, for example in the name Eschscholz, but this is a doubling of the trigraph sch to indicate that the preceding vowel is short rather than itself being a hexagraph.