Many punctuation marks have different uses—think of the comma—but only a few leap off the page to a reader’s eye—as you can tell from this sentence.
A dash can replace a comma to add emphasis—“She was born in Philadelphia—and a beautiful day it was.”
A dash can replace a colon, to introduce the information that follows it—“Her parents always remember her birthday—July 4, 1976.”
A dash can also replace parentheses to indicate, well, parenthetical material—“On the day she was born—it was also the 200th birthday of the United States—her father wore red, white, and blue.”
A dash can be used to indicate the interruption of one thought with another—and its resumption —“When she was twelve she asked—because she was curious—why there were always fireworks on her birthday.”
A dash can indicate an abrupt change—“She was supposed to be a boy—but she wasn’t.”
Note that some dashes travel in pairs, while others go it alone. Style guides specify whether dashes are snuggled right next to the words—as CJR style requires—or whether they get some breathing room — as Associated Press style requires.
Garner’s Modern American Usage calls the dash “perhaps the most underused punctuation mark in American writing.”
But—and this is this columnist’s opinion—it’s frequently overused. This column is an example.
The Elements of Style says the dash “is a mark of separation stronger than a comma, less formal than a colon, and more relaxed than parentheses.” Many writers resort to it, though, without considering whether the sentence is formal or relaxed. And they will sprinkle dashes seemingly willy-nilly, without noticing whether other punctuation marks might make the thought clearer.
To start with, how convoluted is the sentence? If there are a lot of clauses, a pair of dashes might allow a reader to better understand what the sentence is saying. If it’s not convoluted, will a pair of commas, or a set of parentheses, or a colon do?
Then, think about how many dashes you’ve used. They take up a lot of space—the width of a capital M—so you might make an article look shorter on a page by using fewer.
Some writing teachers think the dash is so overused that they all but ban it—that may not be the best course. A good rule of thumb is no more than one dash—or set of dashes—per paragraph, to avoid having an article look like a fill-in-the-blank test. (Garner’s calls for no more than two per sentence: “With three, the reader loses track of what material is part of the main sentence and what is parenthetical.”)
Writers also have to be careful to not mix the single-dash uses with two-dash uses—especially in the same sentence. A sentence like “Clarify your message—you don’t want to bash anyone else to make yourself look better—that’ll bounce back to harm you” clarifies nothing.