The semicolon vs. the em-dash—an eternal quandary.

Few marks of punctuation are misunderstood and misused like these two. Infinitely useful—both connect ideas without the break of a new sentence—yet vaguely understood, both the semicolon and the em-dash tend to be either ignored or overused. When used well, however, they help the flow of ideas like nothing else.

As for me, I’ve used semicolons and em-dashes liberally in my writing since college. Yet, I’ve never paused to consider how they perform similar functions or figure out when to use one over the other—until now.

Similar functions

First, let’s define the correct way to use these marks. I’m only interested in using them to connect thoughts in a sentence, so I’ll omit other uses. I’ll take my direction (and examples) directly from Bryan Garner, in both Modern English Usage and The Chicago Guide to Grammar, Usage, and Punctuation (yes, I own both).

The Semicolon. This is “a kind of supercomma.” It separates sentence parts that need a more distinct break than a comma can signal, but that are too closely connected to make into separate sentences. It is most commonly used to:

  • Unite closely connected sentences. Example: “But Shakespeare’s language appears entirely familiar to us, although it is almost 400 years old; the spelling, the vocabulary, the shapes of the words and phrases seem to have changed but little in that time.”
  • Give a weightier pause than a comma would. This use is discretionary. A comma (or perhaps a dash) would do, but you want a stronger stop. Example: “There is never anything sexy about Lautrec’s art; but there also is never anything deliberately, sarcastically anti-feminist in it.”

The Em-Dash. An em-dash is used to mark an interruption in the structure of a sentence. More specifically, it can be used to:

  • Set off an inserted phrase that, because of what it modifies, needs to go in the middle of a sentence. Example: “In America—as elsewhere—free speech is confined to the dead.”
  • Set off a parenthetical phrase that you want to highlight. Example: “They say—the astrologers, I mean—that it will get better and better for me as I go on.”
  • Tack on an important afterthought. Example: “It was June when we buried him—the summer solstice.”

Garner opines: “The em-dash is perhaps the most underused punctuation mark in American writing. Whatever the type of writing, dashes can often clarify a sentence that is clogged up with commas—or even one that’s otherwise lusterless.”

What’s the difference?

As you can see, both the semicolon and the em-dash introduce a weighty pause to a sentence. I think this is why I’m often unsure of which to use; I know that I want a pause (not a period), and that either a semicolon or an em-dash would produce the right rhythm.

Looking at the uses above, here’s what I observe: an em-dash sets off a thought, while a semicolon unites two closely connected thoughts. I perhaps get into the most trouble by confusing the em-dash’s ability to tack on a thought with the semicolon’s use as a weighty pause.

Even reading Garner has not made this entirely clear to me. Sometimes punctuation is simply an art.

What about you? Is this your quandary, also? Have I used my semicolons and em-dashes correctly in this post?