Writers have three different dashes at their disposal: the hyphen, the en dash, and the em dash.
Most of us are familiar with hyphens and their uses. They’re used to form compound modifiers (such as in “a well-attended event”). We also use them to break a word that falls at the end of a line. This usage is becoming less common, however, because word processing and layout software programs typically have automatic end-of-line hyphenation features.
En and em dashes are less understood. The en dash is the width of a capital N (hence, its name). It’s used to indicate a range, as in the following examples:
For your homework, please read pages 162–195.
The meeting will be on Thursday, 4:00 p.m.–6:00 p.m.
The artist’s blue period, 1948–1952, was his most productive.
En dashes are also used to connect a prefix with an open compound:
post–World War II
The reason for using an en dash with such compounds is to send a subtle signal to the reader that the prefix belongs to the entire compound, not just the first word of it.
Em dashes are the width of (you guessed it) a capital M. Most often, they’re used in pairs to emphasize an element or elements within a sentence:
Cruciferous vegetables—broccoli, cabbage, and cauliflower, for instance—are said to lower the risk of cancer.
Em dashes can also show an abrupt change in thought:
I thought I had time—more than enough time—to catch the train.
Or they can show interrupted dialogue:
“I told you I can’t—”
“You mean you won’t, not can’t,” she said.
Many writers use a double hyphen in place of an em dash, and you might have noticed that your word processor sometimes will automatically turn them into an em dash. Most word processors have shortcuts for creating en and em dashes. It would be worth your while to search your help menu to find out what they are.