Writers need to know how to use the em dash, which is written as one long line (—). This punctuation mark is used to set off a sudden break in thought, an interruption in dialogue, an introductory series, and a parenthetical element (such as an appositive*) in a sentence. Em dashes are often used to set off parenthetical elements within a sentence, like this:
Four states—Illinois, Ohio, Alabama, and New Jersey—are putting up highway signs in metric language.
As you can see, the words between the dashes can be deleted from the sentence without affecting its sense. Dashes used like this are a lot like parentheses, but they are not as strong. A parenthetical phrase is much more of an aside to the reader.
An em dash can also be used to set off a word or words that come at the end of a sentence, like this:
They had twenty-three murders to solve, no leads, and only one suspect—Hannibal Lecter.
As an interruption in dialogue:
“Will he—can he—obtain the necessary signatures?” asked Mr. Smith. Mr. Smith said, “But I thought—”
The em dash should not be used as a generic form of punctuation.
* The term appositive comes from the Latin, “to put near.” An appositive is a noun, noun phrase, or series of nouns placed next to another word or phrase to identify or rename it. Example:
Saint Valentine, the patron saint of lovers, was never married.