E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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 Ms that would stand side by side in the stick. This dictionary is in double columns; each column equals 11 pica Ms 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.