At times, you’ve probably been reading a sentence and come across a set of dashes being used to separate out a phrase or a few words from the main part of the sentence. These dashes — also known as em dashes — are often used incorrectly in writing. Many writers do not know when they should use the em dash; they just throw it in a sentence when it “feels right” or “it looks correct.” But there are guidelines as to when this type of dash should be used.
According to Strunk and White in the Elements of Style, an em dash should be used to “set off an abrupt break or interruption and to announce a long appositive or summary.”
Take the following excerpt from William Faulkner’s short story A Rose for Emily:
When Miss Emily Grierson died, our whole town went to her funeral: the men through a sort of respectful affection for the fallen monument, the women mostly out of curiosity to see the inside of her house, which no one save an old manservant — a combined gardener and cook — had seen in at least ten years.
I bolded the phrase set off by dashes.
Going back to Elements of Style: “a dash is a mark of separation stronger than a comma, less formal than a colon, and more relaxed than a parenthesis.” Though you should only use a dash “when a more common mark of punctuation seems inadequate.”
I’ll leave you with one more example from Faulkner:
That was two years after her father’s death and a short time after her sweetheart — the one we believed would marry her — had deserted her.