The comma might be the most troublesome punctuation mark. This is largely due to the many different ways the comma is used. Sometimes, the comma indicates a pause that would occur if the sentence were spoken aloud. Other times, the comma separates grammatical components of the sentence. Finally, there are mechanical and stylistic uses of the comma that are simply conventional.
The presence or absence of a comma can change the meaning of a sentence—sometimes dramatically. In extreme cases, an erroneous comma can make a sentence mean the exact opposite of what the writer intended. A careful writer must be a careful user of commas.
Conventional or mechanical uses of the comma
There are several uses of the comma that can best be described as conventional or mechanical. The use or omission of the comma is well established, and writers need only to apply the rules.
Most authorities, including The Associated Press Stylebook and The Chicago Manual of Style, recommend a comma after the first digit of a four-digit number. The exceptions include years, page numbers, and street addresses.
We sold 1,270 rare books last year; the most expensive sold for $5,255.
On page 1270 of the report, his address is listed as 5255 Ocean View Drive as of February 5, 2008.
Jr. and Inc.
The trend with these labels is to omit the comma.
David White Jr. is the father of David White III.
He was formerly a senior vice president at Apple Inc.
Degrees and certifications
When a degree or certification is shown after a person’s name, it should be set off with commas.
The report was prepared by Christopher Smith, PhD.
Jane Jones, Esq., has joined the board of directors.
Tom Roberts Jr., MD, FACS, will be the keynote speaker at next year’s conference.
When directly addressing someone, the person’s name or title should be set off with commas.
We could not have done it without you, Lisa.
Thank you, Governor, for your support.
Lori, please stop by my office before you leave for the day.
When a date consists of the day of the month followed by the year, the day of the month should be followed by a comma. When the day of the week is provided before the month, the day of the week should be followed by a comma.
When the date appears in the middle of a sentence, commas should appear both before and after the year.
The store closed its doors for good on Wednesday, October 15, 1958.
Her arrival on Monday, April 11, 1988, was considered a turning point for the company.
When a date is used as an adjective, most authorities require a comma following the year. Yet at least one significant authority (Bryan Garner, in his fourth edition of Garner's Modern English Usage) omits it. Given the uncertainty, it is best to recast the sentence.
The July 10, 2011, meeting was canceled due to a hurricane watch.
The meeting scheduled for July 10, 2011, was canceled due to a hurricane watch.
No comma is used between the month and the year when they are the only two elements in the date.
The store closed its doors for good in October 1958.
The store closed its doors for good in October, 1958.
Her arrival on 11 April 1988 was considered a turning point for the company.
Commas should be used to separate geographic elements, as in the examples below. The final geographic element should also be followed by a comma when it appears in the middle of a sentence.
The mayor of New York was the first guest to arrive; the mayor of Athens, Georgia, was the last to arrive.
His family moved from Bristol, England, to Portsmouth, New Hampshire, when he was eight.
The company is headquartered in Sydney, New South Wales, Australia.
The listing comma
Though not necessarily mechanical, the use of commas in lists is well established. In this usage, the comma separates a series of words, phrases, or independent clauses.
Do not place a comma after the last item in the list (see fourth example below) unless the structure of the sentence otherwise requires it (see third example below, in which the comma after audience is required to separate an introductory dependent clause from the main clause).
For your entree, you may choose vegetarian pasta, beef, chicken, or salmon.
Jane will bring the food, Jose will bring the drinks, John will bring the music, and Jackie will bring the cops.
With dignity, grace, and a tremendous empathy for his audience, he delivered the most moving eulogy.
I am taking art history, Russian literature, microeconomics, and macroeconomics next semester.
Multiple adjectives that modify or describe the same noun
When a noun is modified by more than one adjective, each of which independently modifies the noun, the adjectives should be separated by a comma. In this usage, the comma substitutes for the conjunction and.
The wine offered a fragrant, captivating bouquet.
It was a long, noisy, nauseating flight.
When there are three or more modifying adjectives, it is perfectly acceptable to treat them as a conventional list and include the conjunction and.
It was a long, noisy, and nauseating flight.
If sequential adjectives do not individually modify a noun, they should not be separated by a comma.
He held a bright red balloon.
He held a bright, red balloon.
When an adjective or adverb is repeated for emphasis, a comma is required.
This is a very, very violent movie.
This is a very very violent movie.
Setting off nonrestrictive or nonessential information
After lists, the most important function of the comma is to set off nonrestrictive or nonessential information.
Compare the two sentences below, in which the presence or absence of a comma indicates important information.
I will give the document to my brother, Tom.
Explanation: The writer has only one brother. The brother's name is (grammatically) nonessential and therefore set off with a comma.
I will give the document to my brother Tom.
Explanation: The writer has more than one brother. In this case, the specific brother—Tom—is essential information and should not be set off with a comma.
Nathaniel Hawthorne’s novel The Scarlet Letter has been made into several movies.
Explanation: Hawthorne wrote more than one novel.
Nathaniel Hawthorne’s novel,The Scarlet Letter, has been made into several movies.
Nathaniel Hawthorne’s first novel, Fanshawe, was published anonymously in 1828.
Explanation: Hawthorne had only one first novel.
Nathaniel Hawthorne’s first novel Fanshawe was published anonymously in 1828.
When an explanation or definition occurs as an appositive, it should be set off with commas.
Mary Smith, a staff writer at the Times, recently wrote a book on that subject.
The building’s window placement, referred to by architects as fenestration, is among its most distinctive features.
That and which
These words are frequently misused. That serves as a restrictive pronoun and therefore does not take a comma.
John’s cars that are leased are never kept clean.
Explanation: In this case, the dirty cars are specifically those that John leased; John might have non-leased cars that are kept clean.
Which serves as a nonrestrictive pronoun and therefore requires a comma.
John’s cars, which are leased, are never kept clean.
Explanation: In this case, all of John’s cars are dirty. The fact that those cars are leased is not essential to the meaning of the sentence.
When a nonessential word or phrase occurs in the middle of a sentence, it should be set off with commas.
Your work has been, frankly, awful.
The hotel, once we finally found it, was very nice.
When a word or phrase occurs at the beginning of a sentence, a comma should usually separate it from the main clause.
Yes, we expect to attend the Christmas party.
No, you shouldn’t respond to a rhetorical question.
Honestly, why would you ever think that?
To be honest, I didn’t enjoy the food.
In my opinion, the movie was more compelling than the book.
When a word or phrase follows the main clause at the end of a sentence, it should normally be set off with a comma.
I found the painting rather dull, to be honest.
You will be joining us for dinner, won’t you?
Leave some food for me, please.
We will not be attending the reception, however.
When a sentence ends with an adverb that is essential to the meaning of the sentence, the adverb should not be set off with a comma.
We visited Berlin too.
We took the train instead.
Commas based on sentence structure
This is where things get tricky. Mastering the proper use of the comma in these situations is impossible without at least some understanding of grammar. The rules are easiest to learn and deploy if you first understand four common sentence types: compound, simple, complex, and compound-complex.
A compound sentence contains two or more independent clauses linked by a coordinating conjunction. Independent clauses are those that can stand alone as complete sentences. The most common coordinating conjunctions are and, but, and or. In certain cases, nor, yet, so, and for act as coordinating conjunctions.
Rule: Use a comma before a coordinating conjunction that joins two independent clauses.
She purchased the car, but she declined the extended warranty.
The prime minister’s plan seemed quickly and sloppily put together, and the opposition party immediately attacked it.
Are you traveling in first class, or does your employer limit you to business class on international flights?
I lost my job, so I can’t afford to go to Europe this summer.
Exception to the rule: When the independent clauses are closely connected and short, you may omit the comma.
Elizabeth flew to the conference and Nancy drove.
A simple sentence contains only one independent clause and no dependent clauses. When a simple sentence contains a conjunction, you might be tempted to insert a comma before the conjunction, as you do with a compound sentence. With a simple sentence, however, the general rule is to omit the comma.
Rule: Do not use a comma before a coordinating conjunction if the sentence contains only one independent clause.
She purchased the car but not the extended warranty.
Are you traveling in first class or in business class?
Exception to the rule: If omitting the comma leads to confusion or lack of clarity, insert the comma.
The alumni’s fundraising was better this year than last, and better than expected.
A complex sentence contains an independent clause and one or more dependent clauses. A dependent clause, unlike an independent clause, cannot stand on its own as a complete sentence. The conjunctions and prepositions most commonly used to introduce a dependent clause include if, because, while, as, although, since, and unless.
Rule: If the dependent clause comes before the independent clause, separate it with a comma.
If you can’t see without your glasses, you shouldn’t be driving.
Because of the thunderstorm, our flight has been delayed.
Though I don’t doubt his sincerity, I cannot agree with his position on that issue.
When a sentence begins with two dependent clauses that both apply to the subsequent independent clause, insert only a single comma after the second dependent clause.
If you eat a balanced diet and exercise for a few hours each day, you will feel healthier.
Rule: If the independent clause comes before the dependent clause, omit the comma.
You shouldn’t be driving if you can’t see without your glasses.
Our flight has been delayed because of the thunderstorm.
Exception to the rule: If the dependent clause is not essential to the meaning of the sentence, it should be set off with a comma.
I cannot agree with his position on that issue, though I don’t doubt his sincerity.
Explanation: Not doubting his sincerity is not the reason I cannot agree with his position; it is merely an additional piece of information.
Rule: If the dependent clause occurs in the middle of a sentence, use commas if it is nonessential; do not use commas if it is essential.
The guests, who were all close friends of the president, refused to speak about the events that evening.
The guests who arrived more than an hour late were greeted coolly by the host.
A compound-complex sentence contains two or more independent clauses and at least one dependent clause.
Rule: When a sentence begins with a dependent clause that applies to two independent clauses that follow, insert a comma after the dependent clause, but do not insert a comma between the independent clauses.
If we want this business to work, you need to find suppliers and I need to find buyers.
Rule: When a dependent clause occurs between two independent clauses and applies only to the second, the dependent clause should be set off with commas.
The prime minister’s plan seemed quickly and sloppily put together, and when they saw it, the opposition party immediately attacked it.
When comma rules conflict
When you diligently apply the rules described above, you sometimes end up with a sentence nearly bursting with commas. For example:
I originally wanted to be a rock star, but I decided to become an investment banker.
This is a compound sentence (i.e., one with two independent clauses joined by the conjunction but), the rule for which is to insert a comma before the conjunction.
If you want to add a nonessential comment before the second clause, you end up with this:
I originally wanted to be a rock star, but, in the end, I decided to become an investment banker.
This sentence appears to follow the rules described above. There is a comma before the conjunction (but), and the nonessential comment (in the end) is set off with a pair of commas.
There was a time when this sentence would be punctuated exactly this way. Today, such a sentence is considered over-punctuated. There are two schools of thought on how best to lighten the punctuation of such a sentence.
The traditional approach, advocated by William Strunk Jr. in The Elements of Style, removes the comma after the conjunction. Thus:
I originally wanted to be a rock star, but in the end, I decided to become an investment banker.
With this approach, the commas reflect the natural pausing points if the sentence were to be read aloud. Yet some writers object to this. They argue that whatever is inside a pair of commas should be capable of being removed without turning the sentence into nonsense. In this case, applying that test results in this:
I originally wanted to be a rock star I decided to become an investment banker.
That sentence does not make sense. The contemporary school of thought instead removes the comma before the conjunction:
I originally wanted to be a rock star but, in the end, I decided to become an investment banker.
Now when you remove the comma-bracketed material you get:
I originally wanted to be a rock star but I decided to become an investment banker.
The resulting sentence makes sense. But it’s missing the comma that is ordinarily required before the conjunction linking two independent clauses.
What’s the solution? There are at least two possibilities. The nonessential in the end comment doesn’t really need to be bracketed by commas; the meaning is clear enough without them.
I originally wanted to be a rock star, but in the end I decided to become an investment banker.
The other approach is to introduce a semicolon. This eliminates the troubling conjunction (but).
I originally wanted to be a rock star; in the end, I decided to become an investment banker.