An em is a unit of width in the field of typography, equal to the currently specified point size. For example, one em in a 16-point typeface is 16 points wide. Therefore, this unit is the same for all typefaces at a given point size.
Typographic measurements using this unit are frequently expressed in decimal notation (e.g., 0.7 em) or as fractions of 100 or 1000 (e.g. 70/100 em or 700/1000 em).
Originally, the unit equaled the width of the capital "M" in the typeface and size being used, which gave the unit its name. Under this definition, the length of an em varied with both the typeface and point size.
In metal type, the point size (and hence the em) is measured as the height of the metal body from which the letter rises. In metal type (possible overhangs aside), the physical size of a letter could not normally exceed the em.
In digital type, the em is a grid of arbitrary resolution that is used as the design space of a digital font. Imaging systems, whether for screen or for print, work by scaling the em to a specified point size.
In digital type, the relationship of the height of particular letters to the em is arbitrarily set by the typeface designer. However, as a very rough guideline, an "average" font might have a cap height of 70% of the em, and an x-height of 48% of the em.
Incorrect and alternative definitions
One em was traditionally defined as the width of the capital "M" in the current typeface and point size, as the "M" was commonly cast the full-width of the square "blocks", or "em-quads" (also "mutton-quads"), which are used in printing presses. However, in modern typefaces, the character M is usually somewhat less than one em wide. Moreover, as the term has expanded to include a wider variety of languages and character sets, its meaning has evolved; this has allowed it to include those fonts, typefaces, and character sets which do not include a capital "M", such as Chinese and the Arabic alphabet. Thus, em generally means the point size of the font in question, which is the same as the height of the metal body a font was cast on.
In Cascading Style Sheets, the "em" unit is the height of the font in nominal points or inches. The actual, physical height of the font depends on the user-defined dpi setting and current element font-size.
To make style rules that depend only on the default font size, another unit is developed: the rem. The rem (for “root em”) is the font size of the root element of the document. Unlike the em, which may be different for each element, the rem is constant throughout the document.
An "em-quad" is a metal spacer used in printing presses. It is referred to by this name because it is composed of a square one em on each side. In these old-fashioned printing presses, this allowed the insertion of an em space ( ) character between other typographical characters. It is also occasionally referred to as a "mutton quad".
The width of the em space ( ) is defined to be 1 em, as is the width of the em dash (—). By contrast, the narrower unit en is half an em. Em space is equal to its point size. Suppose a text is in 10pt, then an em space will be of 10pt width.
See standard typographic symbols (mdash, emsp):
- m-width Non-breaking space symbol: UTF8
U+2003 (8195)and HTML emsp. Example: " "
- em dash: UTF8
U+2014 (8212)and HTML mdash. Example: "—"
- Pixels-per-em (PPEm) - used in operating systems to describe the allotment of pixels to em height
- Fullwidth forms
- Non-breaking space width variations
- Bringhurst, Robert (1992). The Elements of Typographic Style. Vancouver, BC: Hartley & Marks. pp. 25–26. ISBN 978-0-88179-033-7. OCLC 25411784.
- This is the traditional definition shown in the Adobe Glossary and the Pocket Oxford Dictionary Third revised edition 1996.
- "W3C. CSS: em, px, pt, cm, in…".
- CSS Length and Units
- The amazing em unit and other best practices
- Typophile Discussion: The Em
- An EM calculator/converter
- Search for origin