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The letter is a type of grapheme, the smallest functional unit within a writing system. Letters are graphemes that broadly correspond to phonemes, the smaller functional units of sound. They are associated with specific names, which may differ between languages and dialects. Letters have a variety of modern uses in mathematics, science, and engineering. The letters were the first words in Old English, replacing the word bōcstæf in the 14th Century. The words "letter" and "word" have been used as an abbreviation since early days of human evolution. The word 'letter' is the first of four letters used to describe a letter (the other four are 'pen,' 'anus' and 'la', 'la-la' and the last two are 'la'