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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 10 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
            first-class–second-class rivalries

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