Em (typography)
Typographic unit equal to the point size
From Wikipedia, the free encyclopedia
An em (from em quadrat) is a unit in the field of typography, equal to the currently specified point size. It corresponds to the body height of the typeface.[1] One em typeface unit is 16 points.[2]
The em space is 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., 70⁄100 em or 700⁄1000 em). The number of pixels per em varies depending on the system.
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 usually exceed the em.
A digital font's design space is called the em, a grid with arbitrary resolution. Scaling the em to a particular point size is how imaging systems—whether for screen or print—work.
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]
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.[4] Possibly, this is because the 'M' sort in such cases cast the full-width of the quad (also known as em quad, mutton quad, or m quadrat); and thus, the width of the sort would equal its point size.[5]
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.[6][7]
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 «rem unit», or root-em, is the font size of the root element of the document. Unlike em, which may vary by element, rem is constant throughout the document.[8]
