|1. Em dash
The chief use of a dash or pair of dashes is to indicate a break in the
flow of thought or an emphatic pause. Do not use three dashes in one
sentence or overuse them on a page. Do not capitalize a complete
sentence that follows a dash.
We were not happy—rather the reverse—at this turn of events.
Then we came to—shall we name the place?—Parkerville.
We enjoyed ourselves immensely—for ten minutes.
Dashes may enclose a phrase that would be set off by commas but which itself contains commas:
There were four of us—Fred, Mary, Joe, and I—by the fire.
No mark of punctuation can stand before a dash except very rarely a
question mark, an exclamation point, a parenthesis, or a bracket.
2. En dash
Use an en dash instead of a hyphen in compound modifiers when one or both of the elements is itself a compound:
Grand Staircase–Escalante National Monument
the Arizona–New Mexico border
Civil War–era house
Some special cases also use an en dash: University of Wisconsin–Milwaukee.
For a range of numbers or dates in display type or in map and graphic
labels, an en dash may be used in place of a hyphen for readability:
20–25 people per square mile
See also HYPHEN, ADJECTIVES AND ADVERBS, PUNCTUATION OF.