The Probert Encyclopaedia of General Information

EM

The em (em-quad) is a unit of horizontal length used in printing, based upon the width of the capital letter 'M' - hence the name. The 12-point em is a standard unit in typography, equal to 1/6 of an inch. An em rule is a horizontal line one em long (a hyphen or dash). In computing, an em is a length as wide as the font size is wide. The em is so named on account of it being equivalent to the width of the body of the letter 'M'.
Research Em