What is 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 called?

em — 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. (Webster's 1913.)

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