"A letter is a segmental symbol of a phonemic writing system. The inventory of all letters forms the alphabet. Letters broadly correspond to phonemes in the spoken form of the language, although there is rarely a consistent, exact correspondence between letters and phonemes.
The word letter, borrowed from Old French letre, entered Middle English around 1200 CE, eventually displacing the native English term bōcstaf (bookstaff). Letter is descended from the Latin littera, which may have descended from the Greek "διφθέρα" (writing tablet), via Etruscan." - (en.wikipedia.org 10.03.2020)
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