When you see a red light, shift down to second. [Without a comma, this sentence
temporarily throws the reader off by seeming to have red lights shifting.]
When you see a red light, shift down to second. [Without a comma, this sentence temporarily throws the reader off by seeming to have red lights shifting.]
BETWEEN INDEPENDENT CLAUSES
Use a comma before a coordinating conjunction (and, but, or, nor, and sometimes so, yet, and for) that links independent clauses.
I had planned to come, but I have changed my mind.
When the independent clauses are short and related, the comma is not necessary but is never wrong.
The power went out and the computer went down.
The power went out, and the computer went down.
Because the conjunctions for, so, and yet can also be other parts of speech, a comma is usually necessary even with short clauses to prevent misreading.
He ran, for the train had started moving.
A semicolon may be used instead of a comma if the independent clauses are long and contain commas or other internal punctuation.
We were assigned two of these [rooms] for the twelve people in our family group; and our official family "number" was enlarged by three digits16 plus the number of this barracks.
Jeanne Wakatsuki and James D. Houston, "Arrival at Manzanar"
AFTER INTRODUCTORY CLAUSES AND PHRASES
A comma usually follows introductory clauses or phrases, signaling where the introductory element ends and the main part of the sentence begins.
Clauses. Use a comma after an introductory adverb clause.
Since the ozone layer gets thinner every year, wearing a good sunscreen is wise.
When the adverb clause comes at the end of the sentence, the two clauses may be separated by a comma if they are only loosely related; however, a comma is not essential.
He should graduate in June, even though he failed some classes.
Phrases. Normally, use a comma after an introductory prepositional or verbal phrase.
During her first year of residency, Dr. Harrison decided she would be a family practitioner, [prepositional phrase]
Looking up from her magazine, Maura smiled at me across the room, [participial phrase]
To enter the contest, you must fill out the registration form, [infinitive phrase]
Finals being over, I began to get ready for the holidays, [absolute phrase]
If an introductory phrase is short and closely related to the main clause, the comma may be omitted unless it is necessary to prevent misreading.
In this context the meaning is different.
TO SEPARATE ITEMS IN SERIES
Three or more words, phrases, or clauses in a series are separated by commas.
The garden was always sunny, cool, and fragrant.
You know you're in trouble when you charge everything, can't pay off your balance every month, and use one credit card to pay off another.
Although the comma before and may be omitted, it sometimes prevents ambiguity. The following book dedication shows how a missing comma can cause confusion or worse.
To my parents, God and Anais Nin.
When one or more of the phrases in a series contains commas, separate the phrases with semicolons.
Among the winners were Amanda Sue Smith, gymnast; Betsy Sue Smith, swimmer; and Stacy Sue Smith, basketball player.
Coordinate Adjectives. When adjectives modifying the same noun can be reversed and still make sense, or when they can be separated by and or or, they should be separated by commas.
She designed a contemporary, two-story, brick-and-glass house.
Do not use a comma after an adjective that modifies a phrase.
He was investigating his damaged garage door opener. [Damaged modifies the phrase garage door opener.]
Never put a comma between a final adjective and the noun it modifies. I was face to face with a huge, angry,/moose.
TO SET OFF NONRESTRICTIVE ELEMENTS
Commas set off nonrestrictive elements, which are clauses, phrases, and words that add information to but do not limit the words they modify. Unlike restrictive elements, nonrestrictive elements can be omitted without changing a sentence's essential meaning. (See restrictive and nonrestrictive elements .)
The new mall, which opened last month, should offer twice as many stores as the old one. [The nonrestrictive clause provides information that is not essential to the meaning of the sentence.]
The mall that is just being built will hold twice as many stores as the mall across town. [The restrictive clause is necessary because it identifies which mall.]
Commas (or, occasionally, dashes or parentheses) signal that an element is nonrestrictive. The meaning of a sentence can depend on whether an element is set off by punctuation, as the following examples illustrate.
She studied longer than anyone else in class, hoping to get an A. [Preceded by a comma, the participial phrase hoping to get an A is nonrestrictive; it simply adds information about why she was studying.]
She studied longer than anyone else in class hoping to get an A. [Without a comma, the participial phrase is restrictive and identifies hopeful classmates.]
Appositives, which are nouns or noun substitutes that rename a nearby noun or noun substitute, can be restrictive or nonrestrictive. A restrictive appositive provides necessary identification and is not set off by commas (my friend Joe; Michael Jackson's album Thriller). A nonrestrictive appositive simply adds information and should be set off with commas (my father, Bill; his first car, a blue Pontiac ).
Parenthetical and Transitional Expressions. Introductory adverbs and transitional expressions (including conjunctive adverbs), contrasting elements, and other sentence modifiers are usually set off by commas.
Unfortunately, winter storms had closed both roads to the ski resort. [adverb] Likewise, the storms had also closed the airport. [transitional expression] Undeterred, he got to his feet again. [participle]
He vowed, however, that he was going to learn to ski no matter how many times he fell.[conjunctive adverb]
The paper was supposed to be twenty pages, not ten. [contrasting phrase]
Commas may be omitted when the word or phrase does not interrupt the continuity of thought or require punctuation for clarity
Perhaps the oversight was intentional. The consequences are nonetheless serious.
A conjunctive adverb or transitional expression (however, nevertheless, consequently, for example, on the other hand) that joins two independent clauses is preceded by a semicolon and followed by a comma.
His punctuation was perfect; however, his spelling was terrible.
Interjections, Direct Address, Tag Questions. Set off mild interjections (such as well, why, oh) and yes and no with commas.
Well, I think you're right.
No, I don't think you should drive.
Use commas to set off a name used in direct address.
Charlie, would you answer the door? Tag questions (shouldn't he? do they?) should be set off by commas. The project was finished on time, wasn't it?
TO SHOW OMISSIONS
A comma sometimes replaces a word or phrase in elliptical constructions. (See also parallel structure.)
Some people choose their clothes for style; others, for comfort. [The comma in the second clause replaces choose their clothes.]
WITH DATES, ADDRESSES, NUMBERS, NAMES, AND TITLES
Dates. In the month-day-year format, separate the day and the year with a comma. In sentences, a comma also follows the year.
Bill Clinton became the forty-second President of the United States on January 20, 1993, at 12 noon.
Do not use commas in the day-month-year format or in dates consisting of only the month and the year.
Bill Clinton became the forty-second President of the United States on 20 January
1993 at 12 noon.
Bill Clinton was elected in November 1992 in a three-way race.
Addresses and Place Names. In text, use commas between the elements of an address (except the state and zip code).
Please send the refund to John Andrews, 11 704 Myers Road , St. Louis , Missouri 63119 . Separate the elements of geographical names with commas. A comma should also set off the state name from the remainder of the sentence.
He was born in Ossian, Indiana, in 1973.
Numbers. Use commas to separate the digits in long numbers into groups of three. A comma in a four-digit number is optional (except in years, page numbers, and street numbers, where a comma is never used).
1,528,200 1528 or 1,528
Names and Titles. Use a comma to separate names that are reversed.
Scissorhands, Edward Use a comma between a person's name and an abbreviated title.
Herbert Morris, Jr. Jane Williamson, Ph.D.
IN CORRESPONDENCE AND DOCUMENTATION
A comma follows the salutation in a personal letter and the complimentar close in both business and personal letters. Use a colon after the salutation in a business letter.
Dear Nancy, [personal letter] Dear Nancy: [business letter] Sincerely yours,
Use commas between certain elements of footnotes and other documentation. (See the Research Paper.)
In a sentence, set off a direct quotation from its attribution with commas.
In 1964, Marshall McLuhan announced, "The medium is the message."
"Don't look back," said baseball great Satchel Paige. "Something may be gaining on you."
Commas are always placed inside quotation marks.
The operator listed the call as "collect," which doubled its cost.
Do not use a comma if the quotation ends with a question mark, an exclamation point, or a dash.
"I am going to quit school!/," he said. "You're going to do what?/," I shrieked. Do not set off indirect quotations with commas.
Andy Warhol predicted,/that everyone will experience fifteen minutes of fame.
TO PREVENT MISREADING
Sometimes a comma is necessary to ensure clarity even though it is not required by any of the principles described previously.
Whatever will be, will be. [Without a comma the repeated verb could cause confusion.]
I mended the vase he had knocked over, and replaced the flowers. [Without a comma between the two verbs, knocked over and replaced might be mistakenly read as a compound verb.]
Do not place a comma between a subject and a verb or between a verb and its object.
The boys in the band/,had a party.
The punch press severed/, his left index finger in one swift motion.
Do not use a comma between the elements of a compound subject or a compound predicate.
The captain of the cheerleaders/,and the leader of the flag team were both juniors. Do not put a comma after a coordinating conjunction, such as and or but.
The coach told them to stop squabbling, but/they continued to berate each other.
Do not place a comma before the first item or after the last item of a series or between cumulative adjectives.
We are considering installing antitheft devices, such as/,window alarms, dead bolts, and hidden/trip wires.
Do not unnecessarily set off a prepositional phrase or separate a concluding adverb clause from the rest of the sentence. Use commas to enclose only nonrestrictive phrases and clauses.
They agreed to meet/,at 3 o'clock/,in the jewelry department. (See also comma splices and run-on sentences .)