See EM in Computer
(Ear & Mouth) Also known as "rEceive & transMit" and "earth & magneto," it is an early telephony term for signaling. For example, PBX operators would signal the long distance operator using E & M. A voltage would be applied to the "mouth" lead of the circuit, which signals the "ear" lead at the other end. There are a variety of two-wire and four-wire E & M signaling methods used between PBXs and PBX to network connections throughout the world.
In typography, a unit of measure equal to the width of the capital letter M in a particular font. The terms "em" and "en" are widely heard with regard to the dashes commonly found in typography, but which are not standard keyboard keys. An "em dash" (one em wide) is used to convey changes such as an abrupt break in thought, a digression or a change from one speaker to another. "En dashes" (width of letter N) are shorter and are used for ranges such as numbers and dates. Contrast with the hyphen, which is a regular text character used to separate telephone and account numbers.
Learn more about EM