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Using an Elizabeth Castro book (HTML, XHTML &CSS), Google (define: em), and Wikipedia I have come up with the following definitions of em, which I feel are practical for web formatting:
- Elizabeth Castro's book, page 124: "An em is usually equal to the element's font size. so 2em would mean "twice the font size".
- A Google definition: "An em is a unit of measurement in the field of typography, equal to the pt size of the current font. This unit is not defined in terms of any specific typeface, and thus is the same for all fonts at a given point size. So, 1 em in a 16 pt typeface is 16 points. This length is also called a firet.
en.wikipedia.org/wiki/Em (typography)"
- A Google definition: "In composition, a unit of measurement exactly as wide and high as the point size being set. So named because the letter "M" in early fonts was usually cast on a square body.
www.compulabel.com/index.asp"
In summary. an em is a square measurement equal to the font size that is in effect. It can be used as a unit of length in cascading style sheets. Regardless of the typeface, it is the same font size for all fonts of a given font size. Historically it relates to the letter M. It is a relative unit of measure (relative to the current font size) that I find handy to use. Using decimals, it can be used with great precision. For example, 3.3ems.
  
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