Anem (fromem quadrat) is aunit in the field oftypography, equal to the currently specifiedpoint size. It corresponds to thebody height of thetypeface.[1] 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.[2]
Theem space is oneem 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 or700⁄1000 em). The number ofpixels per em varies depending on system.
Inmetal type, the point size (and hence theem, fromem 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.
A digital font's design space in digital type is called the em, which is 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 acap height of 70% of the em, and anx-height of 48% of the em.[3]
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' (or 'm')sort in such cases cast the full-width of thequad (also known asem quad,mutton quad, orm 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 (asm orm quadrat), this alternative definition is not used, and also not in many other older texts.[6][7]
InCascading Style Sheets, theem unit is the height of the font in nominal points or inches. The actual, physical height of any given portion of the font depends onthe 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. Therem«rem unit», orroot-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.[8]
Quads were no doubt originally square spaces, as broad as the height of a line. From the fact that the letter m (or M) used to be cast on a square body, such a square space is called an em-quad.
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