The em dash (—) is longer than the en dash (–) and the hyphen (-). It can replace colons, commas, semicolons and parenthesis, and is primarily used in informal writing.
Here are 6 common functions of the em dash:
1. To set off parenthetical information
Espresso—especially when it’s fresh—is the type of coffee that could wake a groundhog in winter.
Em dashes are used instead of parenthesis when the writer wants the parenthetical information to receive a bit more emphasis. Compare these two sentences:
Listening to music—especially rock—is a fantastic way to lift one’s mood.
Listening to music (especially rock) is a fantastic way to lift one’s mood.
In the first sentence, the em dashes add emphasis to the phrase especially rock. In the second example, however, the parenthesis make the added phrase seem less a part of the sentence, and therefore less important.
2. To set off an appositive or appositive phrase
Juan wished his neighbor—a strange man named Marco—would just move out of the neighborhood already.
A writer might choose em dashes, rather than commas, to set off an appositive phrase if doing so results in more clarity:
I was going to work when I saw four men from the gym, Dave, John, Richard and Steve, break into a car.
Although the above sentence is grammatically correct, the number of commas may cause confusion. Here’s the sentence with em dashes:
I was going to work when I saw four men from the gym—Dave, John, Richard and Steve—break into a car.
The em dashes make the sentence easier to read.
3. To add emphasis or elaboration
You’re grades are atrocious—F, F, D, F.
Your house is filthy—full of trash, spoiled food, ants, and cat droppings.
Wow, look at the junk in that milkshake—whipped cream, chocolate syrup, doughnuts!
4. To show a shift in thought
James, before you leave make sure you—oh, never mind. You’re probably not listening.
Why do you always argue with—you know what, I can’t bother with this.
I really liked him when I—well, that was then.
5. To precede quotation attributions
“Be the change that you wish to see in the world.”
—Mahatma Gandhi
“Don’t cry because it’s over, smile because it happened.”
—Dr. Suess
“Always forgive your enemies; nothing annoys them so much.”
—Oscar Wilde
Note
Spaces should not be used before or after the em dash.
Incorrect: I liked the plot — not so much the acting.
Correct: I liked the plot—not so much the acting.
Summary
The em dash is seen most often in colloquial (informal) writing, such as social media posts, magazine articles, blog posts, and in online chat rooms. It should be used carefully—if not sparingly—in formal writing, as it creates a conversational, intimate tone that may not be considered appropriate in formal writing, such as business letters, academic essays and formal reports.