The unit of measure in printing. The standard is a pica M; and the width of a line is measured by the number of such M's that would stand side by side in the “stick.” This dictionary is in double columns, each column equals 11 pica M's in width, and one M is allowed for the space between. Some work is made up to 10 1/2, 20 1/2, etc., ems; and for the half-em printers employ the letter N, which is in width half a letter M. As no letter is wider than the M, and all narrower letters are fractions of it, this letter forms a very convenient standard for printing purposes.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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