Em dashes are abrupt. They stand out, demand recognition, and alert readers that the words to come are important. Em dashes are bold and sometimes unsettling, loudly ushering parenthetical statements into a sentence.
Consider them party crashers—immensely popular ones.
What should the em dash look like?
The examples all show em dashes edged against the words they separate. Two prominent grammar guide books, “The Chicago Manual of Style” and “The Oxford Guide to Style,” advocate this style.
Meanwhile, “The New York Times Manual of Style and Usage” and “Associated Press” both insert spaces between the em dash and the words it separates. PR Daily goes with the Chicago style; either way is fine, just make sure you're consistent.
Despite its absence on computer keyboards, grammarians claim the em dash is overused and misunderstood, that writers have adopted it as catch-all punctuation to replace commas, colons, and semicolons alike.
And they are right. The em dash is wildly misused, probably because it is largely misunderstood. So what is the proper use and why the heck is it called an “em” dash anyway?
Here is a look at grammar’s party crashers.
Formal use required
Younger than the comma, colon, and semicolon, the em dash has evolved into a substitute for all three punctuation marks. Author Lynn Truss notes this phenomenon in her best-selling book on grammar “Eats, Shoots and Leaves.”
“The main reason people use [the em dash], however, is that they know you can't use it wrongly—which for a punctuation mark, is an uncommon virtue,” she writes.
Replacing a colon, semicolon, even a comma with an em dash is permissible for informal writing, like an email to friends. However, grammar experts generally agree that writers should use it sparingly in formal writing.
How to use the em dash correctly
“You use an em dash to set off nonessential clauses in a sentence,” explained Julie DeSilva, former editorial director of ProofReadNow.com. That is the brief, textbook definition and leaves considerable room for interpretation, which perhaps explains why em dash use is so widespread.
Authors of “The Blue Book of Grammar and Punctuation” advise using the em dash for additional emphasis, an abrupt change of thought or an interruption.
The short history of the em dash
As far as punctuation is concerned, dashes are modern; they’re an invention of the 18th century.
While its distant cousin, the hyphen, is more than a 1,000 years old, dashes are the youngsters of the punctuation world. They were introduced in the 18th century and eventually became known as em dashes because of their length. In typesetting, an em dash is the actual length of an uppercase "M," hence the name.
Em dash is twice as long as its sibling the en dash, which is the length of a lowercase "n." Use the en dash to separate numbers in a series. The em and en dashes are not interchangeable.
For instance, “You are the one—the only one—I want to grow old with.” Such additional emphasis requires an em dash. And, as DeSilva mentioned, while “the only one” gives the sentence greater impact, it is ultimately nonessential.
Then there are the abrupt changes or interruptions. For instance, “You are the one I want to grow old with—or so I thought.” Or, “You are the one I want to—oh, never mind.” The latter example is best suited for quoting someone.
And that’s it. The em dash has a very narrow and (some might say) nonessential purpose.
What to use instead
So you’re either rejecting this advice because you are a fan—an undying fan—of the em dash or wondering what’s next.
If you fall into the latter category, the answer is likely the colon, semicolon, or comma. For instance, “I packed numerous items for our trip—swim suit, suntan lotion, mosquito netting, rum, eye patch.” Forget the em dash; replace it with a colon, which is the proper punctuation for announcing this kind of list.
In this case, “The vaccinations are necessary—they will prevent yellow fever.” Drop the em dash and insert a semicolon, because these are two independent clauses related to one another.
Also, the em dash shouldn’t replace the comma. Mark Twain wrote, “Be careless in your dress if you will, but keep a tidy soul” not “Be careless in your dress if you will—but keep a tidy soul.”
Indeed, be careless in your dress, but keep your soul and your sentences tidy.