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Em (typography)
Encyclopedia
An em is a unit of measurement
in the field of typography
, equal to the currently specified point size.
The name of em is related to M. Originally the unit was derived from the width of the capital "M" in the given typeface
.
This unit is the same for all fonts at a given point size.
1 em in a 16 point
typeface is 16 points. Em is frequently expressed in decimal notation, e.g., .7 em, or as fractions of 100 or 1000, e.g. 70/100 em or 700/1000 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.
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 press
es. 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 font
s, 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.
(or CSS
), the W3C best practices recommendations within HTML
and online markup now call for web pages to be based on scalable designs, using a relative unit of measurement (such as the em measurement), rather than a fixed one such as pixel
s ("px") or points
.
es. 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 em dash (—). By contrast, the narrower unit en
is half an em.
Typographic unit
Typographic units are the units of measurement used in typography or typesetting. The traditional units are different from common metric units, as they were established earlier. Even though these units are all very small, across a line of print they add up quickly...
in the field of typography
Typography
Typography is the art and technique of arranging type in order to make language visible. The arrangement of type involves the selection of typefaces, point size, line length, leading , adjusting the spaces between groups of letters and adjusting the space between pairs of letters...
, equal to the currently specified point size.
The name of em is related to M. Originally the unit was derived from the width of the capital "M" in the given typeface
Typeface
In typography, a typeface is the artistic representation or interpretation of characters; it is the way the type looks. Each type is designed and there are thousands of different typefaces in existence, with new ones being developed constantly....
.
This unit is the same for all fonts at a given point size.
1 em in a 16 point
Point (typography)
In typography, a point is the smallest unit of measure, being a subdivision of the larger pica. It is commonly abbreviated as pt. The point has long been the usual unit for measuring font size and leading and other minute items on a printed page....
typeface is 16 points. Em is frequently expressed in decimal notation, e.g., .7 em, or as fractions of 100 or 1000, e.g. 70/100 em or 700/1000 em.
History
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
Cap height
In typography, cap height refers to the height of a capital letter above the baseline for a particular typeface. It specifically refers to the height of capital letters that are flat—such as H or I—as opposed to round letters such as O, or pointed letters like A, both of which may display...
of 70% of the em, and an x-height
X-height
In typography, the x-height or corpus size refers to the distance between the baseline and the mean line in a typeface. Typically, this is the height of the letter x in the font , as well as the u, v, w, and z...
of 48% of the em.
Incorrect and alternative definitions
M
M is the thirteenth letter of the basic modern Latin alphabet.-History:The letter M is derived from the Phoenician Mem, via the Greek Mu . Semitic Mem probably originally pictured water...
" 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 press
Printing press
A printing press is a device for applying pressure to an inked surface resting upon a print medium , thereby transferring the ink...
es. 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 font
Font
In typography, a font is traditionally defined as a quantity of sorts composing a complete character set of a single size and style of a particular typeface...
s, typefaces, and character sets which do not include a capital "M", such as Chinese
Chinese written language
Written Chinese comprises Chinese characters used to represent the Chinese language, and the rules about how they are arranged and punctuated. Chinese characters do not constitute an alphabet or a compact syllabary...
and the Arabic alphabet
Arabic alphabet
The Arabic alphabet or Arabic abjad is the Arabic script as it is codified for writing the Arabic language. It is written from right to left, in a cursive style, and includes 28 letters. Because letters usually stand for consonants, it is classified as an abjad.-Consonants:The Arabic alphabet has...
. 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.
CSS
Online, the use of the em measurement has become more common; with the development of Cascading Style SheetsCascading Style Sheets
Cascading Style Sheets is a style sheet language used to describe the presentation semantics of a document written in a markup language...
(or CSS
Cascading Style Sheets
Cascading Style Sheets is a style sheet language used to describe the presentation semantics of a document written in a markup language...
), the W3C best practices recommendations within HTML
HTML
HyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....
and online markup now call for web pages to be based on scalable designs, using a relative unit of measurement (such as the em measurement), rather than a fixed one such as pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
s ("px") or points
Point (typography)
In typography, a point is the smallest unit of measure, being a subdivision of the larger pica. It is commonly abbreviated as pt. The point has long been the usual unit for measuring font size and leading and other minute items on a printed page....
.
Related terms
An "em-quad" is a metal spacer used in printing pressPrinting press
A printing press is a device for applying pressure to an inked surface resting upon a print medium , thereby transferring the ink...
es. 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
Space (punctuation)
In writing, a space is a blank area devoid of content, serving to separate words, letters, numbers, and punctuation. Conventions for interword and intersentence spaces vary among languages, and in some cases the spacing rules are quite complex....
( ) 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 em dash (—). By contrast, the narrower unit en
En (typography)
An en is a typographic unit, half of the width of an em. By definition, it is equivalent to half of the height of the font . As its name suggests, it is also traditionally the width of a lowercase letter "n"....
is half an em.
See also
- En (typography)En (typography)An en is a typographic unit, half of the width of an em. By definition, it is equivalent to half of the height of the font . As its name suggests, it is also traditionally the width of a lowercase letter "n"....
- Em dash
- TypefaceTypefaceIn typography, a typeface is the artistic representation or interpretation of characters; it is the way the type looks. Each type is designed and there are thousands of different typefaces in existence, with new ones being developed constantly....
- Pixels-per-Em (PPEm) - Used in operating systems to describe the allotment of pixels to Em height.
- x-heightX-heightIn typography, the x-height or corpus size refers to the distance between the baseline and the mean line in a typeface. Typically, this is the height of the letter x in the font , as well as the u, v, w, and z...