The dash ought not to be confused with the hyphen. It is a longer mark, and on a typewriter is made either by two hyphens (—) or by a single hyphen with a space on either side (-).
The dash has no function that is uniquely its own. Instead it acts as a strong comma and as a less formal equivalent to the semicolon, the colon, and the parenthesis. As a substitute for the comma, the dash signals a stronger, more pause. For that reason it should be used sparingly, reserved for occasions when emphasis is really needed.
t> The Dash Isolating Final Constructions
Dashes force an emphatic pause before the last word or phrase of a sentence:
Our time is one of disillusion in our species and a resulting lack of self-confidence—for good historical reasons. Barbara Tuchman
So the gift of symbolism, which is the gift of reason, is at the same time the seat of man's peculiar weakness—the danger of lunacy.
t> The Dash Around Interrupting Phrases and Dependent Clauses Dashes may set off dependent interrupting constructions such as nonrestrictive adjective clauses, adverbial phrases and clauses, appositives, and suspended constructions. In such a use, they create emphasis.
After graduation from high school—where he [Charles Lindbergh] once wrote an elaborate and not uncomical satire on the finicky methods of his English teacher—he took three semesters in engineering at the University of Wisconsin, where the only thing that seemed to interest him much was shooting (he made the rifle team). John Lardner
Occasionally—with a gun in his ribs, another in his back, and a gloating voice saying that in ten seconds he'll be does become a trifle anxious. Charles
Rotten logs can also be host to the ghostly glow of slime fungus, a plant that creeps—glowing—over the logs or along the ground.
Some of those writers who most admired
Henry Adams, and H. G. Wells, for feared it greatly.
Notice, in the last example, that dashes are clearer signals of the grammar than commas would be, since the interrupting series contains commas.
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