Of all the marks of punctuation, the comma (,) has the most uses and is the most misunderstood. To complicate matters, there is more than one acceptable form in a few instances. In the end, the worst comma abuse is overuse. Be consistent in your practice, and when in doubt, leave it out.
1. To separate sentences with two main ideas (compound sentences)
To understand this use of the comma, you need to have studied sentence structure and be able to recognize compound sentences.
When a sentence contains more than two subjects and verbs (clauses) and the two clauses are joined by a connecting word (and, but, or, yet, for, nor), use a comma before the connecting word to show that another clause is coming.
- His first wife tried to poison herself, and his second tried to poison him.
- I thought I knew the poem by heart, but he showed me three lines I had forgotten.
- Are we really interested in helping the children, or are we more concerned with protecting our good names?
If the two parts of the sentence are short and closely related, it is not necessary to use a comma.
- He threw the ball and the dog ran after it.
- Jane played the piano and Charles danced.
Errors to avoid
Be careful not to confuse a compound sentence with a sentence that has a compound verb and a single subject. If the subject is the same for both verbs, you do not need a comma.
- Charles sent some flowers, and wrote a long letter explaining why he had not been able to come. (Unnecessary)
- Last Thursday we went to the concert with Julia, and afterward dined at an old Italian restaurant. (Unnecessary)
- For the third time, the teacher explained that the literacy level of high school students was much lower than it had been in previous years, and, this time, wrote the statistics on the board for everyone to see. (Unnecessary)
2. With a long introductory phrase
Usually if a phrase of more than five or six words precedes the subject at the beginning of a sentence, a comma is used to set it off.
- After last night’s fiasco at the restaurant, she couldn’t bear the thought of looking at him again.
- Whenever I try to talk about politics, my husband leaves the room.
- When it comes to actual facts, every generation makes the same mistakes as the preceding one.
It’s unnecessary to use a comma with a short sentence.
- In January she will go to Switzerland.
- After I rest I’ll feel better.
- At Grandma’s we had a big dinner.
- During the day no one is home.
If an introductory phrase includes a verb form that is being used as another part of speech (a “verbal”), it must be followed by a comma. Try to make sense of the following sentences without commas.
When eating Mary never looked up from her plate. (Unclear)
When eating, Mary never looked up from her plate. (Clear)
Because of her desire to follow her faith in James wavered.
Because of her desire to follow, her faith in James wavered.
Having decided to leave Mary James wrote her a letter.
Having decided to leave Mary, James wrote her a letter.
Above all, common sense is the best guideline when trying to decide whether or not to use a comma after an introductory phrase. Does the comma make the meaning clearer? If it does, use it; if not, omit it.
To set off interrupting material.
There are so many different kinds of interruptions that can occur in a sentence that a list of them all would be lengthy. In general, words and phrases that stop the flow of the sentence or are unnecessary for the main idea are set off by commas. One example is:
3. Before nonrestrictive elements
This rule is not as hard as it sounds. When a modifying word or group of words is not vital to the meaning of the sentence, it is set off by commas. Since it does not restrict meaning of the words it modifies, it is called “nonrestrictive.” Modifiers that are essential to the meaning of the sentence are called “restrictive” and are not set off by commas.
- The girl who wrote the story is my sister. (RESTRICTIVE)
- My sister, the girl who wrote the story, has always been drawn to adventure. (NONRESTRICTIVE)
- John Milton’s renown poem “Paradise Lost” tells a remarkable story. (RESTRICTIVE: Milton has written other poems)
- Dante’s great work, The Divine Comedy, marked the beginning of the Renaissance and the end of the Dark Ages. (NONRESTRICTIVE: Dante wrote only one great work)
- My parakeet Simian has an extensive vocabulary. (RESTRICTIVE: because there are no commas, the writer must have more than one parakeet)
- My parakeet, Simian, has an extensive vocabulary. (NONRESTRICTIVE: the writer must have only one parakeet, whose name is Simian)
- The people who arrived late were not seated. (RESTRICTIVE)
- George, who arrived late, was not seated. (NONRESTRICTIVE)
- She always listened to her sister Jean. (RESTRICTIVE: she has more than one sister)
- She always listened to her husband, Jack. (NONRESTRICTIVE: obviously, she has only one husband)
In the case of which, who or whom:
- NONRESTRICTIVE: Lions, which are mammals, eat meat. (referring to all lions)
- RESTRICTIVE: Lions which have manes are male. (only certain lions) In this case, that is preferable to which.
If a sentence makes sense without the clause, it needs commas. Lions eat meat is true, but Lions are male is not. A nonrestrictive element in the middle of a sentence requires commas before and after it:
- WRONG: Her dad, who is sixty retired early.
- RIGHT: Her dad, who is sixty, retired early.
Conversely, any comma before or after a restrictive element is wrong:
- WRONG: The one on the far left, is my sister.
- RIGHT: The one on the far left is my sister.
All clauses introduced with that are restrictive, as in: My brother attends the same school that I attend.
4. With appositives
An appositive, also called a parenthetical, is a noun phrase equivalent to another noun phrase in the sentence. When it comes at the beginning of a sentence, it needs a comma after it.
- A bold innovator, Josiah Werner is renowned for his many patents.
When it comes in mid-sentence, it requires commas before and after it:
- The Mannes affair was, to put it mildly, a surprise.
- Bathing suits, generally speaking, are getting smaller.
- The chief surgeon, an expert in organ-transplant procedures, took her nephew on a hospital tour.
- Bob, my postman, lives on my street.
- Her name, I think, is Ellen.
- She wore blue, not white, at the wedding. At the wedding she wore blue, not white.
If the appositive is restrictive, it does not need separating commas.
- My classmate Ann plays the flute. (RESTRICTIVE: Ann is not your only classmate)
- Our president, Ron, made a speech. (NONRESTRICTIVE: Ron is your only president)
Don’t confuse restrictive modifying phrases and clauses with interrupting phrases. The following sentences have no commas because the interrupting elements are restrictive:
- I go walking in the park for exercise.
- The girl I think is Ellen is on the left.
5. With transitional words and phrases
- On the other hand, I hope he gets better.
- In addition, the phone rang six times this afternoon.
- I’m, nevertheless, going to the beach on Sunday.
- You’ll find, therefore, no one more loyal to you than I.
- To tell the truth, I don’t know what to believe.
6. Before concluding elements
Use a comma before a concluding phrase or clause if it is not part of the main structure of the sentence and it is nonrestrictive:
- He ran out the door, pausing to kiss his mother.
- I like to run early, when it is still cool.
Careless comma placement can change the meaning of a sentence:
- The boy sent a letter to the girl hoping for news. (the girl hopes for news)
- The boy sent a letter to the girl, hoping for news. (the boy hopes for news)
7. With direct address.
Writers of dialogue sometimes forget to include the comma:
- WRONG: “Mary are you there?” I asked.
- RIGHT: “Mary, are you there?” I asked.
If the name comes in mid-sentence, use a comma before and after it:
- I can’t understand, Mother, what you are trying to say.
- May I ask, Mr. President, why you called us together?
- Physician, heal thyself.
- Thank you, sir.
- Hey, lady, watch out for the car!
8. With yes and no; use exclamation points or commas with interjections
- Yes, it is.
- Oh, I’m so glad to see you.
- Well, have it your way.
- Hey, let me out of here.
- No, I will not let you out.
An error common in dialogue is omission of the comma:
- WRONG: “Yes I am,” she replied.
- RIGHT: “Yes, I am,” she replied.
9. With tag questions (a question that repeats the helping verb and is in the negative)
- I’m really hungry, aren’t you?
- Jerry looks like his father, doesn’t he?
- You’ll come early, won’t you?
- A cup of coffee, please.
10. Between coordinate adjectives (adjectives that modify a noun or pronoun separately)
When more than one adjective (an adjective series) describes a noun, use a comma to separate and emphasize each adjective.
- the long, dark passageway
- another confusing, sleepless night
- an elaborate, complex plan
- the haunting, melodic sound
In these instances, the comma takes the place of “and.” To test if the comma is needed, try inserting “and” between the adjectives in question. If it is logical, it is a coordinating adjective and you should use a comma. The examples below are cumulative adjectives, and would be illogical to separate with commas.
- my usual bad dream
- her long brown hair
11. In a series of three or more
The comma is optional before the and or or, unless it is needed for clarity:
- Aristotle classifies poetry as lyric, dramatic and narrative.
- She sings, dances and acts.
- She lowered the shade, closed the curtain, turned off the light, and went to bed.
Whether or not you omit the comma before the conjunction linking the last item, be sure to keep your usage consistent.
In the following examples, the comma is needed for clarity:
- Would you like to shop at Sak’s, Lord and Taylor’s and Macy’s? (Obscure)
- He got on his horse, tracked a rabbit and a deer and rode on to Canton.
12. To set off contrasting elements.
- Her intelligence, not her beauty, got her the job.
- Your plan will take you further from, rather than closer to, your destination.
- It was a reasonable, though not appealing, idea.
- James wanted an active, not a passive, partner.
13. To prevent confusion. Even when no rule of punctuation requires one, a comma sometimes helps avoid confusing the reader (even if the confusion is momentary):
- CONFUSING: A couple walked in hand in hand.
- CLEAR: A couple walked in, hand in hand.
- CONFUSING: What many people fear does not exist except in their minds.
- CLEAR: What many people fear, does not exist except in their minds.
- CONFUSING: a heavy hooded cloak
- CLEAR: a heavy, hooded cloak or a heavy-hooded cloak
14. Abbreviations after names
- Did you Invite John Paul, Jr., and his sister?
- Martha Harris, Ph.D., will be the speaker tonight.
15. Geographical names and addresses
- The concert will be held in Chicago, Illinois, on August 12.
- They visited Tours, France, last summer.
- The letter was addressed to Ms. Marion Heartwell, 1881 Pine Lane, Palo Alto,
- California 95824. (No comma is used before a zip code.)
16. To set off direct quotations.
Most direct quotes or quoted materials are set off from the rest of the sentence by commas.
- “Please read your part more loudly,” the director insisted.
- “I won’t know what to do,” said Michael, “if you leave me now.”
- The teacher said sternly, “I will not dismiss this class until I have silence.”
- Mark looked up from his work, smiled, and said, “We’ll be with you in a moment.”
Do not to set off indirect quotations or quotes that are used as subjects or complements.
- “To be or not to be” is the famous beginning of a soliloquy in Shakespeare’s Hamlet. (subject)
- Back then my favorite song was “A Summer Place.”(complement)
- She said she would never come back. (indirect quote)
- “Place two tablespoons of chocolate in this pan” were her first words to her apprentice in the kitchen. (subject)
17. In dates.
Both forms of the date are acceptable.
- She will arrive on April 6, 1992.
- He left on 5 December 1990.
- In January 1987 he handed in his resignation.
- In January, 1987, he handed in his resignation.
Spell out from one to nine; numerals from 10 to 999,999; thereafter 1m, 3.2bn (except for people and animals, i.e., 2 million viewers, 8 billion cattle)