Dashes are so easily abused that most discussions of them are sprinkled with warnings and cautions about overuse. That's because inexperienced writers, learning the dash indicates a pause or a shift in the direction of thought, sometimes use it in place of commas and even periods. It's true that the dash should be used sparingly. However, sometimes no other mark will better capture your intended meaning.
1. Use a dash to introduce a base clause summarizing a series of introductory elements.
Example: Murder, armed robbery, extortion—these are all major felonies.
2. Use a pair of dashes to set off a sentence interrupter that contains internal punctuation.
Example: I noticed that the tulips—dusty, faded, slightly rubbery—were not real.
Used this way, dashes show the interrupter as a whole composed of parts. In doing this, they also emphasize the interrupter.
3. Use a dash to attach an afterthought to a sentence that already feels complete.
Example: Last week my daughter phoned to say that she had bought a new pet—a trained armadillo.
Used carefully, the dash can be effective, but use it deliberately. Don't, for example, use a dash to attach trailers to your base clause in a cumulative sentence. The dash creates the feeling that what follows it has been tacked on as an afterthought—a surprise.
4.26 Read the following sentences and comment on the ways in which they do or don't make effective use of the dash. Change sentences that should be changed, and be ready to explain your decisions to the rest of the class.
a. Whenever I eat those green apples, I get sick-really sick.
b. Through several generations of interbreeding, these species of trout, the cutthroat, the rainbow, and the California golden, had become almost indistinguishable.
c. By this time the educational system was already in trouble—the bond issue had been defeated, soundly thrashed.
d. One of the most important thinkers of our century—Ludwig Wittgenstein is almost unknown in this country.
e. The eager young fighter out to make his mark; the ambitious, fast-living girlfriend; the kindhearted trainer-it's a familiar story. f. The local historical society is fighting to preserve that cabin which-they feel-is an important landmark.
g. General Jackson was waiting for a break in the weather—but he was waiting in vain.
h. Flatfooted, overweight, dreamy Milton had no business trying—out for quarterback.
i. The spotted salamander generally makes its home under rotted logs—or among piles of dead leaves.
j. Many people, old, young, and in between, are beginning to wonder about their chances for a secure retirement.