The comma is such a little mark, but it can prompt big confusion -- and heated debate -- about its use.
There's a lot to cover, so let's jump right in. (Believe me, we will not cover everything about commas here, but we'll give it a shot.)
A compound sentence comprises two independent clauses. Each clause has a subject and verb and could stand as a sentence unto itself. It takes a comma before the conjunction (and, but, or, etc.):
The dog barked at a cat, and the cat ran up a tree.
My uncle is sick, but the highway is green.
I can bring you along while I shop for shoes, or you can stay home and watch sports all day. (Decisions, decisions...)
In the following case, the dog is the subject of both verbs, so there should be no comma:
The dog barked at a cat but didn't chase it. (You would use a comma if it were structured this way: The dog barked at a cat, but he didn't chase it.)
Here we have modifying phrases for the cat.
The dog barked at a cat, which then ran up a tree.
The dog barked at the cat that had run up the tree.
(Here, the modifying phrase specifies a particular cat. There may have been other cats, but the pooch barked at the one up the tree. No comma.)
Often we see commas misplaced in sentences with a complex predicate and a modifying phrase:
Wrong: The dog barked at the cat, and for no apparent reason, ate a cantaloupe.
Correct: The dog barked at the cat and, for no apparent reason, sadly ate a cantaloupe.
(The reason should be abundantly clear, of course; the dog is a melon collie.)
Then there is the dreaded comma splice. This occurs when two independent clauses are separated by a comma with no conjunction.
Wrong: The cat barked at the dog, it traumatized the poor pooch.
Either add a conjunction or substitute a semicolon; I prefer the latter.
The cat barked at the dog, and it traumatized the poor pooch.
The cat barked at the dog; it traumatized the poor pooch.
Noun of direct address
Set this off with a comma or two, depending on its placement in the sentence:
Xavier, I'll be out of the office on Tuesday.
Please call me if anything urgent arises, Xavier.
I'll be off Tuesday, Xavier, so please don't do anything stupid in my absence.
In the following cases -- the last two are appositives -- the information set off by a comma or commas is important content but not essential to the structure of the sentence:
Natalie, who does yoga every weekend, has never undergone major surgery.
Godfrey, an amateur taxidermist, dreams of opening a cafe.
An accomplished xylophonist, Darryl frequently is asked to perform at funerals.
When setting off an appositive midsentence -- such as a title or job description -- don't forget the second comma before proceeding with the predicate.
Descriptive versus identifying modifiers
My brother Jeremiah has a unicycle with training wheels. (This presupposes I have more than one brother.)
My sister, Calliope, enjoys writing heroic poetry and playing the steam organ.
Here the commas indicate that I have only one sister.
The same follows for corporate writing:
Gloopnox Corp.'s director of marketing, Leonardo Gloopnox, has been with the company six weeks.
He's the company's one and only marketing director. (He's also the CEO's son.)
Vexco marketing coordinator Lila Langerhans recently announced plans to tweeze her eyebrows.
No commas if she's one among several with that job designation.
Note, too, the following instances:
This summer I'll be traveling to the city where I was born.
This summer I'll be traveling to Walla Walla, where I was born.
The term "the city" needs amplification for the purposes of identification."Walla Walla" could stand on its own, but the phrase that follows the commas delivers supplemental information.
Here we have instances of a dependent clause that precedes and modifies an independent clause:
If I win the lottery, I'll send my whole family to Europe--one-way.
Because the smell of maple syrup makes me queasy, I avoid going into pancake houses.
After Melvin detained Gregory and caused him to be late for his shift in the coal shaft, Melvin was accused of contributing to the delinquency of a miner.
How about the following?
In 2011 the senator voted to filibuster on 53 occasions. In 2005, that same senator called filibusters "obstructionist."
Why the comma in the second instance? I suppose it's a judgment call, but to my mind it emphasizes a contradiction in the senator's two positions; it's shorthand for "however."
Certainly, one might just add "however" and make it explicit:
In 2005, however, that same senator called filibusters "obstructionist."
What follows is a long yet simple series; commas alone can do the job:
I went shopping for eggs, coaxial cable, an electric pencil sharpener, macadamia nuts, a lathe, a pocket calculator, three pounds of figs, a post hole digger, radicchio and a pewter corkscrew.
In the following, there is a series of series, so, for the sake of clarity, the groupings are set off with semicolons:
I went shopping for eggs, radicchio, macadamia nuts and three pounds of figs for tonight's dinner; a post hole digger, coaxial cable and a lathe for some weekend projects; and a pocket calculator, an electric pencil sharpener and a pewter corkscrew to bring to work. (A corkscrew? Judge not, lest ye be judged.)
A series of modifiers
The modifiers can be adverbs:
Garth hastily, sloppily, noisily ate his papaya and smelt sandwich. ("And" could sub in for that second comma, if you like.)
They can also be adjectives:
Maximilian is a loud, uncouth, malodorous, vindictive philatelist.
At least he has a hobby.
There are three fat, brown, male Dachshunds on the porch.
In this sentence, note that the more elemental the trait, the closer it is to what it modifies. The number is most easily changed -- pick up a Dachshund (at your peril, of course) -- and remove it, and the number changes. The doggies could lose weight; that's a variable, too. Brown--well, some pet salons do perform dye jobs. But even if neutered, a male dog remains a he.
Also note that no comma is placed between the number and the other modifiers. In some election stories, you might see this: Five candidates are vying for three, three-year seats on the council. That comma after three is no more necessary than it is in the sentence about the Dachshunds.
Also notice that in this sentence from before -- But even if a male dog is neutered, he remains a he. -- there is no comma immediately after "But."
Some people prefer the serial or Harvard comma; some abhor it. Generally it's unnecessary with a simple series. Garner's Modern American Usage recommends it: "... omitting the final comma may cause ambiguities, whereas including it never will." In business writing, it can come in handy, particularly when the series at hand includes compound elements:
Please notify the people in charge of security, operations, IT, legal affairs, internal and external communications, employee recruitment and retention, research and development and marketing and advertising.
Adding a comma after "development" eliminates potential ambiguity about the number and descriptions of the last entities mentioned.
Please notify the people in charge of security, operations, IT, legal affairs, internal and external communications, employee recruitment and retention, research and development, and marketing and advertising.
Do you come across such contrivances often (other than in articles about punctuation, that is)? Probably not. When you do, diverting from established style for the sake of clarity is fine. You'd rather hear the question, "Egad! Why is Magdalena diverting from her established style on serial commas?" than, "What in heaven's name is Magdalena talking about?"
An either/or question
Do you want what Jay has in the box, or what's behind the curtain where Carol Merrill is standing?
Without the comma, you could answer, "yes" -- meaning you want either of those things.
With the comma, the mandate to choose between the options is implicit: Do you want what Jay has in the box, or [do you want] what's behind the curtain where Carol Merrill is standing?
There's a difference in the way generic and specific utterances are handled, as is shown below:
"All I want in response is a simple 'yes, sir' or 'no, sir,'" Adele said.
"When I ask you if you'd like more tiramisu, just say, 'No, thank you, sir,'" Adele told Vincent.
Sometimes you'll have a quotation that looks like this: "I'm drowsy," Susie said. "And no amount of coffee seems to help."
Susie's statement is a compound sentence, and it should be treated as such: "I'm drowsy," Susie said, "and no amount of coffee seems to help."
It must be all this pedantry about commas. Susie's nearly comma-tose.
OK, we're almost done.
Addresses and dates, cities and states
Mr. Brady went to a party at 148 Bonny Meadow Road, New Rochelle, N.Y.
Oh, what a night I had in December 1963. (No comma between the month and year if a day is not specified.)
On Feb. 29, 2000, I told Randall to take a flying leap.
Velma drove to Tuscaloosa, Ala., and then to Altoona, Pa.
Unless you are writing a formal letter, in which you would use a colon at the end of the salutation -- Dear Sir or Madam: or To the Editor: -- you would use a comma after the recipient's name:
At the end of the letter, a comma would come after the signoff and before your name:
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