parentheses vs. dashes vs. commas
Choosing punctuation to set off supplementary information within a sentence.
Use commas for supplementary information that needs no special emphasis: The proposal, despite its cost, won approval. Use parentheses to make the same information less prominent: The proposal (despite its cost) won approval. Use em dashes to emphasize it: The proposal—despite its cost—won approval. The punctuation sets how prominent the qualification is.
Dashes are especially useful when the inserted material contains commas: Three documents—the lease, the inspection report, and the receipt—were missing. The dashes distinguish the inserted list from the surrounding sentence. Parentheses can enclose the same list when less emphasis is wanted: Three documents (the lease, the inspection report, and the receipt) were missing.
Match the opening and closing marks around an insertion: The manager—usually so patient—interrupted us. Do not open it with a dash and close it with a comma.
When the surrounding sentence continues after the insertion, make sure it remains grammatical without the inserted words. In The repairs—which took a month—were expensive, removing the insertion leaves The repairs were expensive.
Examples
UseThe lecture, originally scheduled for Monday, will take place on Friday. (The commas present the scheduling detail without special emphasis.)
UseThe application (see the instructions on page 4) must be signed. (The parentheses separate a supplementary direction from the main statement.)
UseThe refund—every cent of it—arrived yesterday. (The dashes emphasize that the full amount was returned.)
UseHer requirements—accuracy, discretion, and punctuality—were reasonable. (The dashes enclose a list containing commas.)
AvoidThe director—who had approved the budget, refused to sign. (Close the insertion with a second dash, not a comma.)