Hexagraph

A hexagraph (from the 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 occur in Irish orthography, and many of them can be analysed as a tetragraph followed by the vowels or  on either side to indicate that the neighbouring consonants are palatalized ("slender"). However, not all Irish hexagraphs are analysable that way. The hexagraph, 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 appears in German; for example, in the name Eschscholtz (and thus is the scientific name Eschscholtzia of the California poppy). However, this is a doubling of the trigraph to indicate that the preceding vowel is short rather than itself being a hexagraph.

Irish hexagraphs
is used to write ( in Ulster), e.g. breabhaid "sortie", deabhaidh "haste, skirmish", feabhais "improvement" (genitive case), leabhair "books", meabhair "minds".

is used to write ( in Ulster), e.g. feadhain "troop", Gairmleadhaigh "Gormley" (surname), ghleadhair "struck".

is used to write, e.g. creamhaigh "garlic" (genitive case), sceamhaim "I bark", seamhain "semiology", sleamhain "slippery", teamhair "tor, hill".

and are both used to write  ( in Ulster), e.g. eidheann "ivy", feidheartha "penniless", leigheas "healing", deideigheanna "soft toys", deighealfaidh "will divide".

and are both used to write, e.g. oidheanna "fates", sroigheall "scourge", broigheall "cormorant", oigheann "oven", oighear "ice", poigheachán "(snail)shell".

is used to write, e.g. cheomhair "foggy" (gen.).

is used to write, e.g. ciumhais "edge".