Wikipedia
Heptagraph
A heptagraph is a seven-letter sequence of letters that behaves as a unit and is not the sum of its parts.
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 .