Commas, when well used, can double the clarity and efficiency of writing. This list shows a few ways to use them; please note that there are many exceptions.
Use a Comma:
1) Before a coordinating conjunction (but, and, or, nor, for, so, yet) to join independent clauses (clauses that could be sentences on their own). You can avoid hard-to-read sentences this way:
Confusing: All small boats must return to port immediately or they may sink in the hurricane that is approaching the bay.
Improved: All small boats must return to port immediately, or they may sink in the hurricane that is approaching the bay.
Note: If you used a comma, but omitted the conjunction "or," you have a "comma splice." Don't do this. Never connect two complete sentences with a comma alone (also see Using Semicolons).
Comma splice: All small boats must return to port immediately, they may sink in the hurricane that is approaching the bay.
Correct (with semicolon): All small boats must return to port immediately; they may sink in the hurricane that is approaching the bay.
2) After a short introductory statement.
Unclear: While we were driving a cat crossed the road. (What were you driving? A cat?)
Clear: While we were driving, a cat crossed the road.
Run-on: In Faulkner's novel The Hamlet a character falls in love with a cow that lives on his farm.
Improved: In Faulkner's novel The Hamlet, a character falls in love with a cow that lives on his farm.
3) To set off nonessential, but explanatory, information in a sentence.
The student, in his new BMW, drove into the lake. (the type of car just gives us more information).
The party went on all night, although many guests left quite early.
However: The company wanted an employee who had a lot of computer experience to manage the new system.
(The part stating "who had. . . experience" is essential here).
Rule of thumb: if removing the information would drastically change the sentence's meaning, then that information is essential and should not be set off by commas.
4) Between items in a series.
"He has plundered our seas, ravaged our coasts, burnt our towns, and destroyed the lives of our people." --Thomas Jefferson.
Exceptions and remarks: when the last two items in a series are closely related, you may omit the comma.
The guards brought the prisoner a meal of steak, potatoes, bread and butter.
This rule is controversial, because some grammar books (and professors) will state that the comma must always be used to avoid confusion. In these examples, the issue is not so clear:
The patients sued the insurance company, the hospital and the doctors.
The injured parties sued the insurance company, the hospital, and the doctors.
Were two or three parties being sued? This is where the grammatical rule becomes difficult. When in doubt, ask a professor about her or his "pet peeves."
5) Before and after some transitional words.
This class is, incidentally, only offered at night.
For example, we have to pay the light bill.
He was, moreover, completely unprepared for the test.
Nevertheless, we all got in trouble.
6) To set off quotations from their introductions or closes. Note that a final comma goes inside the closing quotation mark.
"All's well that ends well," wrote Shakespeare.
As Shakespeare put it, "All's well that ends well."
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