Rules for writers, Tenth edition - Diana Hacker, Nancy Sommers 2021

The dash
Other punctuation marks
Punctuation

To use a dash while typing, insert what is called an em-dash using your word processor’s tools or type two hyphens (--). (Most word processing and messaging programs will convert the two hyphens to a dash.) Do not put a space before or after the dash.

Use a dash to set off parenthetical material that deserves emphasis.

Everything in the classroom — from the pencils on the desks to the books on the shelves — was in perfect order.

Appositives, nouns or noun phrases that rename a nearby noun, are ordinarily set off with commas (see 33e). However, when the appositive itself contains commas, use a pair of dashes instead.

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 or 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.

✵ 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 instead use a colon. (See 36a.) The colon is more formal than the dash and not quite as dramatic.

NOTE: Do not overuse the dash. Unnecessary dashes create a choppy effect.