According to The Chicago Manual of Style (16th ed.), there are several kosher uses for the em dash (as opposed to hyphens and en dashes, which will be discussed in a later entry). You can use pairs of them to set off a statement in the same way you’d use parentheses or commas (6.82). You can also use it, as discussed in 6.83, “to set off an introductory noun, or a series of nouns, from a pronoun that introduces the main clause” (as in “Theresa’s blog—it’s an underappreciated masterpiece”). It can also be used to express a sudden break in thought (6.84), or before “that is” or “namely” (6.85), or to start a line of dialog (6.88; rare in American usage).
…and then there’s Emily Dickinson. She breaks all the rules, and that’s why we love her. Check out poem 620 (she did not give her poems formal titles).
Much Madness is divinest Sense—
To a discerning Eye—
Much Sense—the starkest Madness—
’Tis the Majority
In this, as all, prevail—
Assent—and you are sane—
Demur—you’re straightway dangerous—
And handled with a Chain—
Just think of her, shut alone in her room, staring out at the mountains, and thinking to herself, “A comma? Screw that! Written English, it’s time to bend.”
Or a more prim version of that sentiment, anyhow. Her formal experimentation is part of what makes her so modernistic and so expressive. Oddly, though, Dickinson’s daring with punctuation wasn’t widely known until Thomas H. Johnson’s 1955 edition of her poetry. His was the first collection to include her body of work in its entirety, and the first to retain her original style, dashes and all. Dickinson’s reputation grew from obscure to grand over the course of the twentieth century, and Johnson’s effort to let her poetry stand on its own was one of the factors behind her rise. So here’s to the em dash!