Here is a quick guide to using hyphens, en dashes, and em dashes.

First of all, what am I talking about? An em quad is a rectangle as wide as the letter capital M. In typical text fonts, the em quad is square. So an em of 10-point type is normally 10 points high and 10 points wide. An em dash (—) is as wide as an em quad, although in some typefaces, it’s a little shorter than that, with a bit of white on either side to fill out the width. An en quad is half the width of an em quad. And, as you would expect, an en dash (–) is the width of an en quad. A hyphen is typically one-third of an em wide.

  • The hyphen is used to separate parts of a compound modifier. I used a hyphen when I typed “10-point” above. The hyphen (technically the optional hyphen, sometimes called a bell hyphen by old timers who remember the Linotype) is also used to break a word at the end of a line of justified type.
  • The typeset em dash is used to represent the grammatical punctuation mark called a dash. Often, if you are just typing an email or a letter, you might use two hyphens to represent a dash--like that. Some people type space-hyphen-space - like that. Grammatically, a dash indicates a break in thought. It can be represented on the page by an em dash or by a “spaced en dash” (space-en dash-space). The choice is usually a matter of the publisher’s house style. Most American publishers use an em dash. Most British publishers use a spaced en dash.
  • That leaves the en dash, half the width of the em dash, but longer than a hyphen. Where is it used? The en dash has three uses: First, it is used like a hyphen to separate parts of a compound modifier, but only in the case where part of the modifier is an open compound. For example, pre–orld War II has an en dash after “pre” because “World War II” is an open compound. Second, the en dash is used whenever it represents a preposition or conjunction. For example, it is used to replace “to” in ranges like 9–5 and Monday–Friday and it is used to replace “and” in constructions such as the Taft–Hartley Act or chocolate–aspberry torte. The last use is in the spaced en dash, as described above, instead of an em dash.

And what about the minus sign? The minus sign looks like the en dash, which is often substituted. However, the width of the minus sign is based on the width of tabular figures in the typeface, which may be slightly different from an en; and its height is coordinated with that of the plus sign, which may not align with the en dash. So it’s always best to use the true minus sign where it’s called for, rather than an en dash.

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[Brian Jud invited me to contribute short articles on book interior design to his Book Book Marketing Matters newsletter, a biweekly e-zine. This page is adapted from one of those articles.]