In a Time magazine essay, "In Praise of the Humble Comma," author Pico Iyer compares the comma to "a flashing yellow light that asks us only to slow down." But when do we need to flash that light, and when is it better to let the sentence ride on through without interruption? Here we'll consider four main guidelines for using commas effectively. But keep in mind that these are only guidelines: there are no unbreakable rules for using commas--or any other marks of punctuation.
1. Use a Comma Before a Coordinator
Use a comma before a coordinator (and, but, yet, or, nor, for, so) that links two main clauses:
"The optimist thinks that this is the best of all possible worlds, and the pessimist knows it."However, do not use a comma before a coordinator that links two words or phrases:
"You may be disappointed if you fail, but you are doomed if you don't try."
"Jack and Diane sang and danced all night."
2. Use a Comma to Separate Items in a Series
Use a comma between words, phrases, or clauses that appear in a series of three or more:
"You get injected, inspected, detected, infected, neglected, and selected."Notice that in each example a comma appears before but not after the coordinator.
"It is by the goodness of God that in our country we have three unspeakably precious things: freedom of speech, freedom of conscience, and the prudence never to practice either of them."
3. Use a Comma After an Introductory Word Group
"When you get to the end of your rope, tie a knot and hang on."However, if there's no danger of confusing readers, you may omit the comma after a short introductory phrase:
"If at first you don't succeed, failure may be your style."
"At first I thought the challenge was staying awake, so I guzzled venti cappuccinos and 20-ounce Mountain Dews."
4. Use a Pair of Commas to Set Off Interruptions
Use a pair of commas to set off words, phrases, or clauses that interrupt a sentence:
"Words are, of course, the most powerful drug used by mankind."But don't use commas to set off words that directly affect the essential meaning of the sentence:
"Literature is all, or mostly, about sex."
"Your manuscript is both good and original. But the part that is good is not original, and the part that is original is not good."
Also see the discussion of restrictive and nonrestrictive clauses at Building Sentences with Adjective Clauses.
For practice in applying these four guidelines, see: