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8. Use a dash to set off an abrupt break or interruption and to announce a long appositive or summary.
A dash is a mark of separation stronger than a comma, less formal than a colon, and more relaxed than parentheses.
His first thought on getting out of bed—if he had any thought at all—was to get back in again.
The rear axle began to make a noise—a grinding, chattering, teeth-gritting rasp.
The increasing reluctance of the sun to rise, the extra nip in the breeze, the patter of shed leaves dropping—all evidences of fall drifting into winter were clearer each day.
Use a dash only when a more common mark of punctuation seems inadequate.
Her father’s suspicions proved well-founded—it was not Edward she cared for—it was San Francisco.
Her father suspicions proved well-founded. It was not Edward she cared for, it was San Francisco.
Violence—the kind you see on television—is not honestly violent—there lies its harm.
Violence, the kind you see on television, is not honestly violent. There lies its harm.
Excerpted from: Strunk, William Jr., and E.B. White. The Elements of Style, Fourth Edition. New York: Longman, 2000.
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