The em is a relative unit of measurement equal to the size of the font in use: at 12 points, an em measures 12 points. Originally, it was a small square block of lead, with no letter, slipped into the composition to create a space.
Where to see it
The em gives its name to the em dash (—), the longest of the dashes, used in French to introduce lines of dialogue. It is also the reference unit in layout software, where sidebearings and tracking are set in thousandths of an em, and on the web with the CSS em unit.
Why this detail matters
Knowing the em makes it possible to set spacing precisely:
- Because it depends on the font size, the em keeps the same proportions whatever the size of the text, which makes it an ideal unit for spacing;
- Tracking and kerning are expressed in thousandths of an em in layout software, a value of 10 corresponding to one hundredth of the font size;
- In CSS, the em unit follows the same principle and makes it possible to build interfaces whose spacing scales with the text.
Related term
The en is a unit equal to half an em, which gives its name to the en dash.