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Commas arguably have become the most widely misused punctuation mark.  Generally, use a comma if one of the rules below calls for it.  You can find these rules, along with more examples, in Andrea Lunsford's The Everyday Writer, 5th edition (with exercises), pp. 400-412.  For guidance on the three most common punctuation errors we see in the Writing Center -- comma splices, fused (run-on sentences), and sentence fragments -- see pp. 385-396 of the same volume.  The Writing Center has a copy of The Everyday Writer for you to peruse.


1.   Use commas to set off introductory words, phrases, and clauses.

In fact, healthcare costs keep rising.

Wearing new shoes, Brendan prepared for the race.

While the storm was raging, we read by candlelight.

(Note that some writers omit the comma after an introductory element if it is short and does not seem to require a pause after it.)


2.   Use commas with conjunctions that join clauses in compound sentences.  A comma usually precedes a coordinating conjunction (for, and, nor, but, or, yet, so) that joins two independent clauses in a compound sentence. 

The show started late, and the crowd grew quiet.

The title sounds impressive, but administrative assistant clerk is just another word for photocopier.

(The comma can sometimes be omitted between short clauses.)


3.     Use commas to set off nonrestrictive elements.  Nonrestrictive elements are clauses, phrases, and words that do not limit, or restrict, the meaning of the words they modify.  Since such elements are not essential to the meaning of the sentence, they should be set off with commas.

The two drivers involved in the accident, who have been convicted of drunken driving, should lose their licenses.

On the other hand, if a clause, phrase, or word does restrict the meaning of the words it modifies, it is essential to the meaning of the sentence, and so we do not place commas around it.  

Drivers who have been convicted of drunken driving should lose their licenses.


4.     Use commas with items in a series.

He has plundered our seas, ravaged our coasts, burnt our towns, and destroyed the lives of our people.

-- The Declaration of Independence

All the cafeteria's vegetables -- broccoli, green beans, peas, and carrots -- were cooked to a gray mush.

(The final comma in a series, often called the serial comma or Oxford comma, is often omitted in space-restricted formats such as newspapers.)

(When the items in a series contain commas of their own or other punctuation, separate them with semicolons rather than commas:  Should I serve kidney beans, which are red; cranberry beans, which are white and red; or chick peas, which are tan?)


5.    Use commas to set off parenthetical and transitional expressions.

Some studies, incidentally, have shown that chocolate, of all things, helps prevent tooth decay.

Ozone is a by-product of dry-cleaning, for example.


6.     Use commas to set off contrasting elements, interjections, direct address, and tag questions.

Remember, sir, that you are under oath.

The governor did not veto the unemployment bill, did she?


7.    Use commas with dates, addresses, titles, and numbers.

The attacks on the morning of Tuesday, September 11, 2001, took the United States by surprise.

Forward my mail to the Department of English, The Ohio State University, Columbus, Ohio 43210.

Portland, Oregon, is much larger than Portland, Maine.

Oliver Sacks, MD, has written about the way the mind works.

The city's population rose to 158,000 in the 2000 census.

(Do not use commas with dates in inverted order or with dates consisting of only the month and the year:  She dated the letter 26 August 2008.  Thousands of Germans swarmed over the wall in November 1989.  The titles Jr. and Sr. often appear without commas.  The comma is optional within numerals of four digits but never occurs is four-digit dates, street addresses, or page numbers.)


8.    Use commas to set off most quotations.

A German proverb warns, "Go to law for a sheep, and lose your cow."

"All I know about grammar," said Joan Didion, "is its infinite power."

(Do not use a comma when you introduce a quotation with that:  The writer of Ecclesiastes concludes that "all is vanity."  Do not use a comma after a question mark or exclamation point:  "Out, damned spot!" cries Lady MacBeth.)


9.    Use commas to prevent confusion.

The members of the dance troupe strutted in, in matching costumes.

Before, I had planned to major in biology.


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