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An em (from English '' em quadrat'') is a
In metal type, the point size (and hence the ''em'', from '' em quadrat'') 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
In some older texts, but not all, the em is defined, or said to have been defined, as the width of the capital 'M' in the current typeface and point size.This alternative definition is shown in th
Adobe Glossary
and the Pocket Oxford Dictionary Third revised edition 1996. Possibly, this is because the 'M' (or 'm') sort in such cases cast the full-width of the
CSS Length and Units
An EM calculator/converter
The Formula of converting PX to EM
{{Typography Display technology Typography Units of length
unit
Unit may refer to:
Arts and entertainment
* UNIT, a fictional military organization in the science fiction television series ''Doctor Who''
* Unit of action, a discrete piece of action (or beat) in a theatrical presentation
Music
* ''Unit'' (a ...
in the field of typography
Typography is the art and technique of arranging type to make written language legible, readable and appealing when displayed. The arrangement of type involves selecting typefaces, point sizes, line lengths, line-spacing ( leading), an ...
, equal to the currently specified point size. For example, one em in a 16-point typeface
A typeface (or font family) is the design of lettering that can include variations in size, weight (e.g. bold), slope (e.g. italic), width (e.g. condensed), and so on. Each of these variations of the typeface is a font.
There are thousands o ...
is 16 points. Therefore, this unit is the same for all typefaces at a given point size.
The em dash
The dash is a punctuation mark consisting of a long horizontal line. It is similar in appearance to the hyphen but is longer and sometimes higher from the baseline. The most common versions are the endash , generally longer than the hyphen b ...
and em space
Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consi ...
are each one ''em'' wide.
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., em or em).
History
In metal type, the point size (and hence the ''em'', from '' em quadrat'') 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
In typography, cap height is the height of a capital letter above the baseline for a particular typeface
A typeface (or font family) is the design of lettering that can include variations in size, weight (e.g. bold), slope (e.g. italic), wi ...
of 70% of the em, and an x-height
upright 2.0, alt=A diagram showing the line terms used in typography
In typography, the x-height, or corpus size, is the distance between the baseline and the mean line of lowercase letters in a typeface. Typically, this is the height of the le ...
of 48% of the em.
Obsolete alternative definition
In some older texts, but not all, the em is defined, or said to have been defined, as the width of the capital 'M' in the current typeface and point size.This alternative definition is shown in thAdobe Glossary
and the Pocket Oxford Dictionary Third revised edition 1996. Possibly, this is because the 'M' (or 'm') sort in such cases cast the full-width of the
quad
Quad as a word or prefix usually means 'four'. It may refer to:
Government
* Quadrilateral Security Dialogue, a strategic security dialogue between Australia, India, Japan, and the United States
* Quadrilateral group, an informal group which inc ...
(also known as ''em quad'', ''mutton quad'', or ''m quadrat''); and thus, the width of the sort would equal its point size.
Note however, that in the oldest attested English text from 1683 mentioning em (as ''m'' or m quadrat), this alternative definition is not used, and also not in many other older texts.
CSS
InCascading Style Sheets
Cascading Style Sheets (CSS) is a style sheet language used for describing the presentation of a document written in a markup language such as HTML or XML (including XML dialects such as SVG, MathML or XHTML). CSS is a cornerstone tec ...
, 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.
See also
*Em dash
The dash is a punctuation mark consisting of a long horizontal line. It is similar in appearance to the hyphen but is longer and sometimes higher from the baseline. The most common versions are the endash , generally longer than the hyphen b ...
(—)
* En (typography)
* Fullwidth forms
* List of XML and HTML character entity references
* Non-breaking space width variations
* Pixels-per-em (PPEm) – Used in operating systems to describe the allotment of pixels to em height
* Responsive web design
* Whitespace characters
In computer programming, whitespace is any character or series of characters that represent horizontal or vertical space in typography. When rendered, a whitespace character does not correspond to a visible mark, but typically does occupy an ...
References
External links
CSS Length and Units
An EM calculator/converter
The Formula of converting PX to EM
{{Typography Display technology Typography Units of length