em dash
The em dash (—) is the long dash that sets off words in a sentence with a strong, deliberate break.
The em dash does the work of a comma, a pair of commas, or a colon, but with a stronger break.
A pair of em dashes encloses nonessential material that interrupts a sentence: The farmhouse—built in 1890—still stands at the end of the road. A question mark or exclamation point that belongs to the enclosed material stays inside the dashes: Her explanation—was she serious?—left us puzzled.
A single em dash sets off a word, phrase, list, or clause that ends a sentence. It does the work of a comma when the closing words add emphasis or an afterthought: She had been to Paris—twice. It does the work of a colon when the closing words explain, define, or itemize the idea before them: The kit held three tools—a hammer, a wrench, and a screwdriver. We could not enter—the locks had been changed. A single em dash can also follow an opening list that the rest of the sentence sums up: Rent, utilities, insurance—every bill arrived the same week.
In dialogue, an em dash marks speech that is cut off; an ellipsis marks speech that trails off.
Use no more than one pair of em dashes in a sentence, because a second pair makes it hard to tell which dashes belong together.
When a sentence holds two sets of nonessential material, enclose one in em dashes and the other in commas or parentheses, so the reader can tell the sets apart: The report, due Friday, covers the merger—a deal worth millions—and its effects. Each use of the em dash has its own entry, listed under Em Dash.
Examples
UseOur neighbors—both retired teachers—run the garden club. (A pair of dashes encloses nonessential material.)
UseThe storm left us without one thing—power. (A single dash adds emphasis.)
UseThe figures did not agree—one invoice had been counted twice. (A single dash does the work of a colon and introduces an explanation.)
UseA passport, a ticket, a change of clothes—that was all she brought. (A single dash follows an opening list that the rest of the sentence sums up.)
AvoidWe kept only the essentials—food and water—. (Delete the second dash.)