Notwithstanding its versatility, the em dash is best limited to two appearances per sentence. Otherwise, confusion rather than clarity is likely to result.
A pair of em dashes can be used in place of commas to enhance readability. Note, however, that dashes are always more emphatic than commas.
And yet, when the car was finally delivered—nearly three months after it was ordered—she decided she no longer wanted it, leaving the dealer with an oddly equipped car that would be difficult to sell.
A pair of em dashes can replace a pair of parentheses. Dashes are considered less formal than parentheses; they are also more intrusive. If you want to draw attention to the parenthetical content, use dashes. If you want to include the parenthetical content more subtly, use parentheses.
Note that when dashes are used in place of parentheses, surrounding punctuation should be omitted. Compare the following examples.
Upon discovering the errors (all 124 of them), the publisher immediately recalled the books.
Upon discovering the errors—all 124 of them—the publisher immediately recalled the books.
When used in place of parentheses at the end of a sentence, only a single dash is used.
After three weeks on set, the cast was fed up with his direction (or, rather, lack of direction).
After three weeks on set, the cast was fed up with his direction—or, rather, lack of direction.
The em dash can be used in place of a colon when you want to emphasize the conclusion of your sentence. The dash is less formal than the colon.
After months of deliberation, the jurors reached a unanimous verdict—guilty.
The white sand, the warm water, the sparkling sun—this is what brought them to Hawaii.
Two em dashes can be used to indicate missing portions of a word, whether unknown or intentionally omitted.
Mr. J—— testified that the defendant yelled, “Die, a——,” before pulling the trigger.
From the faded and water-damaged note, we made out only this: “Was ne——y going to m—— K——, but now ——t.”
When an entire word is missing, either two or three em dashes can be used. Whichever length you choose, use it consistently throughout your document. Surrounding punctuation should be placed as usual.
The juvenile defendant, ———, was arraigned yesterday.
The em dash is typically used without spaces on either side, and that is the style used in this guide. Most newspapers, however, set the em dash off with a single space on each side.
Most newspapers — and all that follow AP style — insert a space before and after the em dash.
Many modern word processors will automatically insert an em dash when you type a pair of hyphens. Otherwise, look for an “insert symbol” command. If you are using a typewriter, a pair of hyphens is the closest you can get to an em dash.