Both dashes and parentheses are used to set off interrupting comments, explanations, examples, and other similar parenthetical elements from the main thought of the sentence. Commas are ordinarily used when parenthetical or other nonrestrictive elements area closely related in the main thought of the sentence. Dashes and parentheses are used when the interruption is abrupt and the element set off is only loosely related to the main thought of the sentence.

Though the choice between dashes and parentheses is sometimes a matter of taste, dashes emphasize more strongly the element being set off and give it greater importance than parentheses do. Parentheses are more commonly used when the element enclosed is an incidental explanatory comment, an aside, or a nonessential bit of information.

A single dash is used following an introductory element or preceding a final sentence element. A pair of dashes is used to enclose an element within a sentence. Parentheses are always used in pairs around the enclosed element. In handwriting, distinguish the dash from the hyphen by making the dash longer. In computer written copy, use two hyphens without spacing to indicate the dash. Example: word--word.


Use the dash or a pair of dashes to mark an abrupt shift in sentence structure or thought. 

Could she–should she even try to–borrow money from her aunt?

The Queen of England never carries money–too unseemly–but travels with ladies in waiting who pay from the royal purse for whatever Her Majesty fancies.

That puppy is going to grow up to be enormous–check out the size of his paws–and will eat us out of house and home.

Use the dash to set off nonrestrictive appositives and other parenthetical elements for emphasis.

Indeed, the crassest form of more--is better using an easily measured quantity to assess improperly a far more subtle and elusive quality–is still with us.

In any fight against the sun, man–for all his technology–will come out the loser.

Use the dash for clarity to set off internally punctuated appositives or other parenthetical elements. To prevent confusion, use dashes rather than commas to set off appositives containing punctuated items in a series.

In the following sentence the word object appears to be one item in a series.

Putting a spin on an object, a top, a bullet, a satellite, gives it balance and stability.

But when the commas are replaced by dashes, the meaning is clear.

Putting a spin on an object–a top, a bullet, a satellite–gives it balance and stability.

Use the dash to set off introductory lists or summary statements.

Gather data, tabulate, annotate, classify–the process seemed endless to the research assistant.


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