# Em (typography)

An em is a unit in the field of typography, equal to the currently specified point size. For example, one em in a 16-point typeface is 16 points. Therefore, this unit is the same for all typefaces at a given point size.[1]

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).

The name "em" was originally a reference to the width of the the capital "M" in the typeface and size being used, which was often the same as the point size.[2]

## History

A metal sort. The line height c is the precursor of the em.

In metal type, the point size (and hence the em) was equal to the line height of the metal body from which the letter rises. In metal type, 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.[3]

### Incorrect and alternative definitions

The letter "M", on the left in Perpetua and on the right in Calisto, inside squares of one em on each side.

Although the size of the em ultimately depended on the point size, or height of the metal body of a letter, it is generally used as a measure of horizontal spacing, with vertical spacing being measured in picas or points.[1] One em was traditionally defined as the width of the capital "M" in the current typeface and point size,[2] because the "M" was commonly cast the full-width of the square "blocks", or "em-quads" (also "mutton-quads"), which are used in printing presses.

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.

## CSS

In Cascading Style Sheets, the "em" unit is the height of the font in nominal points or inches. The actual, physical height of any given portion of the font depends on the user-defined dpi setting, current element font-size, and the particular font being used.

To make style rules that depend only on the default font size, another unit was developed: the rem. The rem, or 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.[4]

## Related terms

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 old-fashioned printing presses, this allowed the insertion of an em space ( ) between other type. 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. In 10 pt text, an em space will be 10 pt wide.

## Associated Symbols

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: "—"