In CSS, an' em' is what is referred to as a relative unit, like percentages. Good design allows scaling and uses relative size values; it almost always provides excellent implementation of ems. An 'em' is equal to the computed value of the 'font-size' property of the element on which it is based. The exception is when an 'em' occurs in the value of the 'font-size' property itself, in which case it refers to the font size of the parent element. It may be used for vertical or horizontal measurement.
In a scalable font the "em" is subdivided into units. In a postscript font there are usually 1000 units to the em. In a TrueType font there might be 512, 1024 or 2048 units to the em. In an Ikarus font there are 15,000 units. PfaEdit uses these units as the basis of its coordinate system.
Dimensionless distance measuring units used in Macromedia's software program Fontographer.