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This is an excerpt that started with my naïve attempt this spring at defining The Em for the wikidpedia, where someone called Mishka, well, never mind. The only other example I have of this problem, is in the "a bomb" (not A-bomb) and "the bomb" (as in a-bomb) situation, where "everyone" understands the difference...;)
...the type part is so "simple," it's best to concentrate on that, even if you have to start from....well, literally, there is "the origin," which is unfortunately not a good place to start!... What is the em? Not an em, or a em, ems, em quads or m&ms!...THE EM?
A. "An em is a unit of measurement in the field of typography. The unit is defined in the terms of a specific typeface, and thus varies in length." That Wikipedia! EM, can't phix it, can't quill it!
B. "The em is a sliding measure. One em is a distance equal to the type size. In 6 point type, an em is 6 points; in 12 point type an em is 12 points and in 60 point type an em is 60 points." well okay( Bringhurst), but that's both "The em, one em, & an em" in one definition.
C. "An em is the width of the cap height." E. Lupton
D. With all due respect, some of, some of the above ;)
"An Em" is the horizontal measure equal to the point size being composed in typesetting. When "an Em" is used as a spacing measure, it is shorthand for — " 'Typesetter'; give me a space along the line of text equal to 1 times the point size, please!" The vertical distance a typesetter moves, when the next line of text is desired, e.g. a carriage return, is usually the point size, plus some; like 12 point type with 2 points of leading, known as "12 on 14", this indicates that even though the type is 12 pt, the typesetter has moved 14 points down to start the next line. When the Typesetter moves down with no additional space, (known as "no leading", or 12 on 12), the vertical move is an em. If you move down two ems and start the next line, then it is double line spacing, e.g. 12 on 24.
The horizontal moves made by a typesetting system, including, an "em space", or an "em dash", are defined on a glyph-by-glyph, and font-by-font basis, ("I" being narrower than "M", or the em dash of a very wide face being wider than the em dash of a very narrow face, respectively, e.g. and not based on the "cap height"). If the font is a mono-spaced design, all horizontal moves are the same space, including an em.
All "That" uses the term "em" in the context of a given size during composition, and there it means "whatever the point size is times X" , so yes, "an em" is a sliding measure, but it is so because of the shorthanded nature of the command, and... it is not The Em. "The Em" is the amount of space on which a typeface design is converted to a font with the knowledge:
"This vertical space
represents the distance
the typesetting system
will move when the next
line below is located with
no extra 'leading' between
the lines of text."
This definition works for wood, stone, metal, film, bitmaps, and outlines. Modern font formats have definitions of ascent and descent from a baseline. As the total of these serve as the definition of the vertical move that used to be the Em, then that total is the new em.
Comments are welcome, or better yet, if you already agree, go hound Wiki for us...