Em (typography)

In metal type, the physical size of a letter could not normally exceed the em.

Scaling the em to a particular point size is how imaging systems—whether for screen or print—work.

In digital type, the relationship of the height of particular letters to the em is arbitrarily set by the typeface designer.

[5][6] In Cascading Style Sheets, the em unit is the height of the font in nominal points or inches.

To make style rules that depend only on the default font size, another unit was developed: the rem.

A metal sort . The line height, c , is the precursor of the em.
The letter M , on the left in Perpetua and on the right in Calisto , inside squares of one em on each side. The M in Perpetua does not match the point size of the em.