There’s a great article by Peter K. Sheerin on alistapart for those out there who care about both web and typographic standards. Well worth taking a look at, but watch out for some sub-editorial quirks (remember the twenty percent language gap between ourselves and our American brethren) before you quote the article as gospel.
Hyphens are not dashes.
Stop! Go back and re-read the subhead above–at least 2–3 times then let it sink in before continuing.
The sentence above illustrates the proper use of the hyphen and the two main types of dashes. They are not the same, and must not be confused with each other. In some fancy fonts the difference is more than just the width–hyphens have a distinct serif. If you don’t know the rules already, let’s review them. First, though, a definition:
An “em” is a unit of measurement defined as the point size of the font. 12 point type uses a 12 point “em.” An “en” is one-half of an “em.”
Though some of the finer points in the rules are complex, their basic applications are clear-cut and their misuse easily identifiable. First, neither an em dash nor an en dash should be confused with the hyphen (-), which is used to join compound words together. (continued)
Reprinted here without express permission.