The dash is a versatile punctuation mark that signals a rupture in a sentence’s logic or syntax. It often sets off an amplifying or explanatory element within a clause. Its role can be similar to that of commas, parentheses, or colons, but the dash conveys a sharper emphasis than its cousins, underscoring the disruptive nature of what follows. It often signals an abrupt shift in tone or break in thought.

Surely he’d be home soon—but no, it was karaoke night!

You may use a dash to introduce an expression, amplifying phrase, series, or appositive.

There’s only one song I’ll sing—“Brother, Can You Spare a Dime?”

Dashes may also set off an emphatic aside or a parenthetical expression from the rest of the sentence.

And then—despite the fact that I had already lost my voice!—I tried to get on stage and sing a Beyoncé song.

Note that the interjected phrase contains its own internal punctuation in the preceding sentence. A question mark or exclamation point may punctuate interruptive phrases set off by dashes.