A heptagraph is a sequence of seven letters used to represent a single sound (phoneme), or a combination of sounds, that do not correspond to the individual values of the letters.
Heptagraphs are extremely rare. Morse code uses a heptagraph, ⟨· · · — · · —⟩, for the dollar sign. Most other fixed sequences of seven letters are composed of shorter multigraphs with a predictable result. The seven-letter German sequence ⟨schtsch⟩, used to transliterate the Russian letter ⟨щ⟩, as in ⟨Borschtsch⟩ [bɔʁʃt͡ʃ] for Russian ⟨борщ⟩ [borɕː] "borscht", is a sequence of a trigraph ⟨sch⟩ [ʃ] and a tetragraph ⟨tsch⟩ [t͡ʃ]. Likewise, the Juu languages have been claimed to have a heptagraph ⟨dtsʼkxʼ⟩, but this is also a sequence, of ⟨dtsʼ⟩ and ⟨kxʼ⟩.