= Em
{disambiguate=typography}
{title2=$\mathrm{em}$}
{wiki}
= Em
{synonym}
An <em> is a nominal typographic length set by the current <font size>. At a <font size> of $S$ <typographic points>, one <em> is $S$ <typographic points> long.<A Glossary of Typographic Terms by Adobe>[Entry em, em space, em quad: the current point size]{ref} A scalable <computer font> describes its geometry relative to this scale. In <OpenType>, `unitsPerEm` specifies how many <font units> make one <em>.<Font Header Table by Microsoft>[head table, unitsPerEm]{ref}
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.<CSS Fonts Module Level 4 by W3C>[Section2.5, note about arbitrary glyph ink bounds relative to the EM box]{ref} <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.
Back to article page
