An em is a nominal typographic length set by the current font size. At a font size of typographic points, one em is typographic points long.[ref] A scalable computer font describes its geometry relative to this scale. In OpenType,
unitsPerEm specifies how many font units make one em.[ref]Font Header Table by Microsoft. head table, unitsPerEm
Suppose one em contains 2048 font units. A design length of 1024 font units is half an em. At a font size of 12 typographic points, that length is 6 typographic points; at 18 typographic points, it is 9 typographic points. The stored coordinate has not changed; the scale has.
The word em does not require the glyph for “M” to be exactly one em wide. Nor is an em the measured ink height of every glyph: a shape can occupy only part of the nominal square or extend beyond it.[ref]
CSS Fonts Module Level 4 by W3C. Section2.5, note about arbitrary glyph ink bounds relative to the EM box
Glyph advances, left side bearings and right side bearings must consequently be read from the computer font's metrics rather than inferred from the em alone. Articles by others on the same topic
In typography, an "em" is a unit of measurement that is used to define the size of type and the spacing between characters and lines. The term originates from the width of the capital letter "M" in a given typeface, though the actual size of an em varies depending on the font size being used. 1. **Relational Measurement**: The em unit is considered a scalable measurement.