This is a place to post articles and have discussions of the most misunderstood of all punctuation.
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The correct way
Em-dash has two—and only two—legitimate uses, as is acknowledged by all punctuational pedants. Well, by me anyway. Well by me right now, who knows what I’ll say tomorrow.
- To create a dramatic pause setting aside a concluding word or phrase that is marked for special or contrary emphasis. In this use it is similar to a colon. Start by using a colon, but make it an em-dash if you find that it needs to go up to eleven. It’s like a colon in a tutu.
- To set aside a phrase internal to the sentence—like this—that is more distinctly separate than it would be using commas. It’s like commas with social distancing.
It also has an illegitimate use—I guess—or does it—where it acts as a—kind of glitch or stutter in text that is meant to be disjointed.
no some cases an em-dash will take a space on either side. It’s wide enough. It depends on the typeface and publishing context: newspapers — and some digital publishing platforms based on the newspaper style or featuring truly awful typefaces — will expect spaces. However, spaces will look weird in any well-designed typeface (like this one: see above). A prudent em-dasher, knowing what really matters, would of course avoid such disreputable publishers.
If you’re Emily Dickinson, you can do what you want.