When typing, use two hyphens to form a dash (--). Do not put spaces before or after the dash. If your word processing program has what is known as an “em-dash” (—), you may use it instead, with no space before or after it.
Use a dash to set off parenthetical material that deserves emphasis.
Everything that went wrong—from the peeping Tom at her window last night to my head-on collision today—we blamed on our move.
Use a dash to set off appositives that contain commas. An appositive is a noun or noun phrase that renames a nearby noun. Ordinarily most appositives are set off with commas (32e), but when the appositive itself contains commas, a pair of dashes helps readers see the relative importance of all the pauses.
In my hometown, people’s basic needs—food, clothing, and shelter—are less costly than in a big city like Los Angeles.
A dash can also be used to introduce a list, a restatement, an amplification, or a dramatic shift in tone or thought.
Along the wall are the bulk liquids—sesame seed oil, honey, safflower oil, and that half-liquid “peanuts only” peanut butter.
In his last semester, Peter tried to pay more attention to his priorities—applying to graduate school and getting financial aid.
Everywhere we looked there were little kids—a bag of Skittles in one hand and their mommy or daddy’s sleeve in the other.
Kiere took a few steps back, came running full speed, kicked a mighty kick—and missed the ball.
In the first two examples, the writer could also use a colon. (See 35a.) The colon is more formal than the dash and not quite as dramatic.
tip: Unless there is a specific reason for using the dash, avoid it. Unnecessary dashes create a choppy effect.