Heptagraph
A heptagraph is a sequence of seven letters used to represent a single sound, 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, used to transliterate the Russian letter, as in for Russian "borscht", is a sequence of a trigraph and a tetragraph . Likewise, the Juu languages have been claimed to have a heptagraph, but this is also a sequence, of and.
