One em in a 16 point typeface is 16 points. 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.
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 of 70% of the em, and an x-height of 48% of the em.
Incorrect and alternative definitions
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 presses. 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 fonts, 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.
Online, the use of the em measurement has become more common; with the development of Cascading Style Sheets (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 pixels ("px") or points.
An "em-quad" is a metal spacer used in printing presses. 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".
- Pixels-per-Em (PPEm) - Used in operating systems to describe the allotment of pixels to Em height.
- Em in CSS - examples
- The amazing em unit and other best practices
- Typophile Discussion: The Em
- Em Calculator (for CSS Layouts)
- An EM calculator/converter
- The em-quad (square)
- Search for origin