em(n = noun.quantity) em quad, mutton quad - a quad with a square body; "since `em quad' is hard to distinguish from `en quad', printers sometimes called it a `mutton quad'"
The portion of a line formerly occupied by the letter m, then a square type, used as a unit by which to measure the amount of printed matter on a page; the square of the body of a type. [1913 Webster]
Em, n. Printing 1 a unit for measuring the amount of printed matter in a line, usually equal to the nominal width of capital M.
2 a unit of measurement equal to 12 points.
Idiom
em rule (or dash) a long dash used in punctuation.
Etymology
name of the letter M
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