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A is for
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The A-Z of typographic terms.
monotype.com

In text figures, the shape and positioning of the numerals vary as those of lowercase letters do. In the most common scheme, 0 , 1 , and 2 are of x-height, having neither ascenders nor descenders; 6 and 8 have ascenders; and 3 , 4 , 5 , 7 , and 9 have descenders.