Many people put commas in simply where they "sound good": where you think you would pause if reading the sentence aloud. While there is some flexibility in the rules governing commas where you can use your own judgment or "ear," there are specific rules that dictate when a comma should be used. Since the comma is often critical in helping the reader to understand your sentence, you should know these rules and be able to use them.
There are eight main rules governing the use of commas, and we'll review those here.
A. When two independent clauses are linked by a conjunction, a comma precedes the conjunction.
Before you get confused, let me explain what an independent clause is. An independent clause has both a subject and a predicate. It could stand by itself as a complete sentence (so it's called independent).
A conjunction is a linking word used to connect words, phrases, clauses, or sentences. The conjunctions we're talking about here are coordinating conjunctions, that link phrases of equal weight. They are the following: and, but, or, for, nor, so, yet. Here is an example.
He approached the counter, and the clerk told him the store was closed.
Both clauses-- He approached the counter and the clerk told him the store was closed--are independent. Each one has a subject and predicate. Each one could stand on its own as a complete sentence. But instead they are joined with a conjunction, and. Therefore, according to our first rule, we need a comma before the and. Here are a few more examples.
The movie ended, but the audience didn't leave.
He ate the entire cake, for he didn't want to share it.
You can either go on the blind date, or you can sit home and mope.
One common error is to insert the comma even when there are not two independent clauses.
He went to the store, and bought a cake.
In this example, bought a cake is not an independent clause. It has no subject. It is a dependent clause. It cannot exist by itself. Therefore there should be no comma.
A complication occurs when the second independent clause begins with an introductory element. For example,
We went to the movies, and after that, we went to dinner.
Again, we have two independent clauses, We went to the movies and we went to dinner. They are joined by a conjunction, and. But the second clause begins with an introductory element, after that. You may be tempted to put a comma after and. Don't do it. We still need the comma before and, as we've discussed above. And to set this introductory element apart, a comma after that is sufficient.
B. Most introductory elements are followed by a comma.
Notice the word most here. Don't you love the English language? There are several different types of introductory elements. They include adverbs, conjunctive adverbs, transitional expressions, adverb clauses, participles, infinitives, and other types of phrases. Luckily, it's not really necessary to know exactly what each of these are. The main thing is that you recognize when you are using an introductory element.
In part A, we had two independent clauses of equal weight in one sentence. Here, we have an introductory element (it may be a word, phrase, or dependent clause) opening the sentence, followed by an independent clause (the main clause). Here's an example.
When a criminal is put to death, the whole society is guilty of murder.
The first part of the sentence, When a criminal is put to death, is an introductory element modifying the main clause, the whole society is guilty of murder. It tells us when the main clause is true. Here are some more examples.
As we all know, murder is wrong.
For example, long illnesses often cause much pain.
On the other hand, there are drugs to help.
A comma after an introductory element is never wrong. However--and here's the tricky part--you can omit it after a short introductory element if the omission does not create confusion. For example,
After class we should meet for lunch.
By twenty careers of models are often over.
Both examples can have commas, the first after class and the second after twenty. Omitting the comma in the first case still allows for an easily understandable sentence, so this is okay. Omitting the comma in the second case creates a very confusing sentence. This is not acceptable.
C. Use commas between items in a series.
A series consists of three or more items of equal importance. These may be words, phrases, or clauses. Here are some examples.
The iguana was large, green, and scaly.
He bought her roses, candy, and jewelry.
If he's home, if he's in a good mood, and if he's already eaten, then you can ask to borrow the car.
Greg got his degree in engineering, worked at Bell Labs, and invented the automatic tooth-brusher.
When one or more of the items in the series contains a comma, you should separate the items with semicolons instead of commas (see Part 2, Section B of this essay).
D. Use commas to separate coordinate adjectives.
Adjectives that equally modify the same word are called coordinate adjectives. These should be separated by a coordinating conjunction or by a comma. For example,
The sleek, black cat wound its way around her legs.
The sleek and black cat wound its way around her legs.
It was a dark, silent, empty room.
You can tell these are coordinate adjectives because you could pair each adjective separately with the noun and it would make sense (sleek cat, black cat, dark room, silent room, empty room).
When the adjectives are cumulative and not coordinate, do not use commas. Adjectives are cumulative when the one nearer the noun is more closely related to the noun in meaning. For example,
The dark red dress was her favorite.
Here each adjective does not modify the noun equally. If you took out red and simply said dark dress, it would change the meaning entirely. Dark modifies red, not dress. These adjectives are working cumulatively. Therefore, we don't use a comma.
If you're not sure if adjectives are coordinate or cumulative, try changing their order or putting the word and between them. If you can do this and it still makes sense, then you have coordinate adjectives. If you apply this to our example above, you can clearly see these are cumulative adjectives.
The red dark dress was her favorite.
The dark and red dress was her favorite.
Here's another example.
He likes to watch action-packed science fiction movies.
Here again the adjectives are cumulative. Action-packed modifies science fiction, not movies.
E. Use commas to set off nonrestrictive elements.
A restrictive element limits the meaning of the words it modifies. It helps to define or tell which one.
A nonrestrictive element doesn't limit the meaning of the words it modifies. It simply gives additional, non-essential information. Here are some examples:
Students who use drugs should be expelled.
The clerk, who had worked there for only a day, fumbled with the change.
In the first example, who use drugs is a restrictive element. If you take it out of the sentence, it changes the meaning of the sentence. It is necessary to tell us which students should be expelled. Because it is critical to the sentence, it is restrictive and is not set off by commas.
In the second example, who had worked there for only a day is a nonrestrictive element. If you take it out of the sentence, the meaning of the sentence remains the same. The nonrestrictive element simply adds some additional, non-critical information about the clerk. Because it is nonrestrictive, it is set off by putting a comma before and after.
This can sometimes get a little confusing. Say you have one brother named John.
Your brother, John, ate my ice cream.
This is correct. John is nonrestrictive because even if we take it out of the sentence, we still know whom the sentence is about. You only have one brother.
But what if you have three brothers? In that case, the sentence above is incorrect. John then becomes an essential piece of information telling us which brother ate the ice cream. John then becomes a restrictive element. And so the sentence should be punctuated as follows:
Your brother John ate my ice cream.
Remember that these elements may also come at the end of a sentence.
I will meet with Barbara Smith, who owns the ice cream parlor.
F. Use commas to set off other nonessential elements.
Like nonrestrictive elements, many other elements are not essential to the meaning of a sentence but add to its tone or texture. These nonessential elements, unlike nonrestrictive elements, don't refer to any specific word in the sentence. These nonessential elements include parenthetical expressions, absolute constructions, phrases of contrast, tag questions, direct address, mild interjections, and yes and no. But again, you don't need to know exactly what all these are as long as you understand the overall rule. Here are some examples.
He was, in fact, the largest green iguana in captivity.
His size, surprisingly, was over six feet long.
To tell the truth, I was a bit shocked when I saw him.
His mouth twisted in a grimace, he lowered his hands into the iguana cage.
The experiment having succeeded, the lab now wanted to raise the largest cockroach in captivity.
The iguana needs less food, more exercise.
The iguana, not the dog, was the biggest.
You didn't like the movie, did you?
They're not too exciting, are they?
Maggie, you have to try this cake.
Oh, you're such a chicken.
Well, if you never try it, you'll never know.
Yes, the iguana is pretty.
No, I don't want to kiss it.
G. Use commas with dates, addresses, place names, numbers, and titles.
This is pretty self-explanatory. Here are some examples showing exactly where the commas go in dates, addresses, and places.
My first date with Allen was April 5, 1992, and he asked me to marry him April 6, 1992.
The iguana was born in October 1991.
I live at 123 Faux Road, Photown, New Hampshire 03043.
Manchester, New Hampshire, is the largest city in three states.
Use commas with numbers of four or more digits, but do not use commas within a date, street numbers, telephone numbers, Social Security numbers or ZIP codes.
The iguana weighs 1,225 pounds.
My phone number is 603-555-5555.
I've heard this lecture 5,333,333 times before.
Use commas to set off a title that follows a name. Note that if the title includes a period, that period comes before the comma.
Barbara Smith, Ph.D., didn't know how to do her laundry.
John Bell, Jr., has taken over his father's plumbing business.
H. Use commas with quotations.
With direct quotations, expressions such as she said, he asked, and so on are set off by commas. Here are some examples.
"Now, when we get over there," Pullman said, "we'll have a couple of drinks."
"You're doing the right thing!" Sonny cried.
Billy said, "I hate green, scaly things."
"Knowledge is power," wrote Francis Bacon.
Exercises for Part 1
Now here are some exercises for you to do covering all the rules governing the comma. Insert commas where necessary.
- His wife Barbara was terminally ill.
- He went to his desk which was in the corner by the window.
- Her father his anger gone sat meekly in his chair.
- Saint Anselm in fact is one of the top colleges in the country.
- Freshmen who live off campus have a lower grade point average.
- Jane's iguana a huge creature had escaped from its tank and was sunning itself on the patio.
- When Jane feeds him in the morning he runs over to his dish but if she hasn't given him dog food he won't eat anything.
- He approached the counter and the clerk told him the store was closed.
- The party roommate is loud obnoxious and rude.
- When I sit down at the computer my mind goes blank.
- As almost all students know grammar was invented by evil teachers.
- In the year 2295 iguanas have taken over the Earth.
- My favorite foods are pizza ice cream and iguana.
- He went to the store and bought a cake.
- She jumped high grabbed the rope and swung across the room.
- "I got a call from Julie" Vinnie said.
- Jane asked "What did Julie say?"
- "She asked me" Vinnie said "if I have ever owned an iguana."
- You can order an iguana by writing to Reptile City P.O. Box 255 Chicago Illinois 20209.
- You're obsessed with iguanas aren't you?
- No my dear I simply recognize their superior intellect.
- A college that stresses the liberal arts is superior.
- Everyone was shocked when the World Trade Towers the tallest buildings in New York were bombed.
- I wish John that you would pay more attention.
- The droning nasal voice of my professor put me to sleep.
- The golden brown color of the toast made my mouth water.
- John was ready to scream cry and pull his hair out all at once. Then he realized he'd finished the comma exercises.