em quad

The story

Attested in 1841. From 1808 em quadrat, from 1683 m quadrat, from m + quadrat, a metal type, which is as wide as the point size of the font. The origin of the letter m is unclear; perhaps because the type body of the letter M was approximately square during the early days of typesetting, and to contrast it with the n quadrat, the letter n being approximately half as wide.