On Writing Well: The Classic Guide to Writing Nonfiction

by William Zinsser

The Dash. Somehow this invaluable tool is widely regarded as not quite proper—a bumpkin at the genteel dinner table of good English. But it has full membership and will get you out of many tight corners. The dash is used in two ways. One is to amplify or justify in the second part of the sentence a thought you stated in the first part. “We decided to keep going—it was only 100 miles more and we could get there in time for dinner.” By its very shape the dash pushes the sentence ahead and explains why they decided to keep going. The other use involves two dashes, which set apart a parenthetical thought within a longer sentence. “She told me to get in the car—she had been after me all summer to have a haircut—and we drove silently into town.” An explanatory detail that might otherwise have required a separate sentence is neatly dispatched along the way.