Punctuation is a convention of writing that helps readers more clearly understand written language. There are ten punctuation rules for using commas as punctuation marks in written American English:
- Separating items in a series
- Separating verb clauses
- After adverbials that precede the main clause
- Separating coordinate adjectives
- Separating nonrestrictive adjective clauses
- Separating nonessential appositives
- Separating contrasted coordinate phrases
- Separating nonessential adverbs and parentheticals
- Transitioning between quotations
- In dates, geographical places, numbers, personal titles, direct addresses, and brief interjections
The following sections explain and provide examples of the punctuation rules for commas in written English.
Use a comma to separate items in a series. A series is a list of three or more items. Place a comma after each item but the last in the series including before the coordinating conjunction. The seven coordinating conjunctions in English are and, but, for, nor, or, so, and yet. For example:
- Cinnamon, nutmeg, and cloves are my favorite spices.
- The little boy runs, jumps, bikes, and swims all summer.
- Your bedroom is dusty, disorganized, and unsanitary.
- Farmers grow red cherries, orange pumpkins, yellow corn, and green peppers.
Do not use a comma to separate items in lists of only two items. (Note that some styles do not require the serial comma.)
Between Verb Clauses
Use a comma to separate two or more verb clauses that are joined by a coordinating conjunction. A verb clause — which is also referred to as an independent clause, main clause, superordinate clause, or matrix clause — is a clause that contains both a subject and a predicate and that functions as a complete sentence. For example:
- My mother cooked the turkey, but I prepared the sweet potatoes.
- He came, he saw, and he conquered.
- Some preview students were lost on campus, so a professor offered to give them a tour.
- Some vegetarians eat fish, but others refuse to eat any meat.
Use a comma after an adverbial in the form of a prepositional phrase, verb phrase, or adverb clause that precedes the verb clause. The three types of adverbials in English are the adjunct adverbial, disjunct adverbial, and conjunct adverbial.
Adjunct adverbials are words, phrases, and clauses that modify an entire clause by providing additional information about time, place, manner, condition, purpose, reason, result, and concession. Disjunct adverbials are words and phrases that provide additional information to frame an entire clause and that denote the attitude of the speaker toward or judgment of the proposition such as truthfulness of manner of speaking. Conjunct adverbials are words and phrases that express textual relationships and that serve to link clauses. For example:
- To learn to write in English, students must study punctuation.
- Because of the storm, the power was out all over town.
- Although she hated camping, she agreed to go because she loved her husband.
- In my opinion, writing well requires one to have studied grammar.
Generally do not use a comma before an adverbial or adjunct that follows the main clause.
Between Coordinate Adjectives
Use a comma between coordinate adjectives. Coordinate adjectives are defined as adjective phrases that equally describe one noun and can be separated by the coordinating conjunction and. For example:
- The guitarist played a beautiful, haunting melody.
- The explorer travelled on a narrow, treacherous road up the mountain.
- Our neighbors own a loud, rambunctious dog.
- My brother is a stubborn, difficult, unruly child.
Do not use a comma between cumulative adjectives. Cumulative adjectives are defined as adjective phrases that cannot be connected by the coordinating conjunction and and whose meaning are cumulative.
Nonrestrictive Adjective Clauses
Use a comma to separate a nonrestrictive adjective clause. A nonrestrictive adjective clause is defined as an adjective clause that is optional and whose addition or deletion does not change the meaning of the main clause. For example:
- The book, which happens to be my favorite, is available at the library.
- My mother, who happens to be sitting by the window, is an avid reader.
- The castle, where we got married, was burglarized last night.
- She lived in Delhi, which is the capital of India.
Do not use a commas to separate a restrictive adjective clause. Also note that the same adjective clause may be either restrictive or nonrestrictive depending on context.
Use commas to separate a nonrestrictive appositive. A nonrestrictive appositive is defined as an appositive that is optional and whose addition or deletion does not change the meaning of the main clause. For example:
- My uncle, the man with the guitar, got us tickets to the concert.
- My mom, a nurse with an advanced degree, will attend the birth of my baby.
- The winner, you, must report any additional earnings.
- My daughter, Poppy, is a toddler now.
Do not use commas to separate a restrictive appositive. Also note that the same appositive may be either restrictive or nonrestrictive depending on context.
Use a comma to indicate thousands, millions, billions, and so on in higher numbers. Place a comma every three digits. For example:
- Our new car cost $29,000.
- The population of the United States of America is approximately 308,878,000 people.
- Milner Library owns roughly 1,500,000 volumes.
- Scientists estimate that Earth is 3,550,000,000 years old.
Do not use a comma in place of a decimal point in written American English.
Use a comma to separate the day of the week from the month, the day from the year, and the year from the rest of the sentence. For example:
- On September 11, 2001, the United States changed forever.
- Abraham Lincoln was born on Sunday, February 12, 1809, in Kentucky.
- She was born on November 26, 1920.
- The final date of the sale is Tuesday, January 3, 2010.
Do not use a comma when the day precedes the month or between months and years or seasons and years.
Use a comma between street addresses and city names, city names and state names, and state names and countries. For example:
- The majority of my students are from Peoria, Illinois.
- The White House is located at 1600 Pennsylvania Avenue, Washington, D.C.
- My mother-in-law once lived in Quebec, Canada.
- Bloomington, Indiana can be quite the party town.
Do not use a comma after a single part location.
Use a comma to separate a personal title that follows a name. For example:
- Maria Lopez, Vice President for Finances, works in the administration building.
- Pamela Brown, M.D., is a highly reputable obstetrician.
- Martin Luther King, Jr. believed in equality.
- Marvin Cheshire, Sr. worked at the company for eighteen years.
Do not use a comma to separate a personal title that precedes a name. Also note that some styles no longer require the use of a comma before Jr. and Sr.
Use a comma to separate a direct address from the rest of the sentence. For example:
- Jenny, would you please open the window?
- Do you know where the pens are, Daniel?
- Lynne, clean your room.
- You, Ms. Maggie Hovey, have won the grand prize!
Do not use a comma to separate an indirect address from the rest of the sentence.
Use a comma to separate a brief interjection from the rest of the sentence. For example:
- The teacher said that, yes, there will be an essay question on the final exam.
- You haven’t seen my new coat, have you?
- No, the café is not open on Christmas.
- The pub is having a St. Patrick’s Day party, of course!
Punctuation is a convention of writing that help readers more clearly understand written language. Commas perform ten basic functions in written American English: separating items in a series; separating verb clauses; after adverbials that precede the main clause; separating coordinate adjectives; separating nonrestrictive adjective clauses; separating nonessential appositives; separating contrasted coordinate phrases; separating nonessential adverbs and parentheticals; transitioning between quotations; and in dates, geographical places, numbers, personal titles, direct addresses, and brief interjections.
Note that different styles (MLA, APA, Chicago, LSA) have varying rules for punctuation marks including commas. Always check your style guide for the rules specific to your subject area.
Faigley, Lester. 2003. The Brief Penguin Handbook. New York: Pearson Longman.
Gibaldi, Joseph. 2003. MLA Handbook for Writers of Research Papers. 6th ed. New York: The Modern Language Association of America.