How to Use Commas Before Coordinating Conjunctions

A coordinating conjunction is a word that connects different parts of a sentence. Commas must be placed before coordinating conjunctions that separate independent clauses—word groups that can function as complete sentences.

• Example: The mother went to the store, but the child went to school.

In the above example, “The mother went to the store” is an independent clause, and “the child went to school” is also an independent clause. Both word groups can function as complete sentences. For this reason, a comma is placed before the coordinating conjunction but.

Do not use a comma before a coordinating conjunction if it belongs to a word group that is not an independent clause.

• Example: The mother went to the store but not the bank.

In the above example, “The mother went to the store” is an independent clause, but the phrase “not the bank” is not an independent clause. Therefore, the phrase "not the bank" cannot function as a complete sentence. For this reason, a comma is not placed before the coordinating conjunction but.

How to Use Commas with Items in a Series

Commas for two items in a series

Do not use a comma if a sentence contains a list of only two items.

• Example: I like apples and bananas.
• Example: I like to paint and sing.

Do not use a comma if a sentence contains a list of only two items, even if each item is described with many words.

• Example: I like apples baked with walnuts and bananas topped with chocolate and whipped cream.

In the above example, no commas are required. Even if many words are being used to describe items in a list, from a grammatical standpoint there are only two items being listed: 1. apples [baked with walnuts] and 2. bananas [topped with chocolate and whipped cream].

Commas for three or more items in a series

Use commas to separate three or more items in a series.

• Example: The café's smoothie flavors are apple, mango, strawberry, and banana.
• Example: The café's smoothie flavors are apple, mango, strawberry and banana.

In the first example, a comma is placed before the last item in the list, and banana. In the second example, a comma is not placed before the last item in the list. Both sentences are grammatically correct. A comma that comes before the last item in a list is known as a serial comma or Oxford comma.

The Oxford comma is used to make sure that lists are not confusing. In the example above, without the use of the Oxford comma, a reader may think that the café sells a strawberry and banana smoothie, when the café actually sells a strawberry smoothie and a banana smoothie.

In MLA style, always use the Oxford comma.

Commas for two adjectives in a series

If two adjectives modify the same word, separate them with a comma only if the adjectives are coordinate. A set of coordinate adjectives are adjectives of equal importance.

• Example: The hot, humid day seemed to drag on forever.
• Example: The humid, hot day seemed to drag on forever.

In the above examples, both pairs of adjectives are coordinate because they modify the same nouns and are of equal importance.

A set of two coordinate adjectives can also be separated by the word and rather than by a comma.

• Example: The hot and humid day seemed to drag on forever.
• Example: The humid and hot day seemed to drag on forever.

If the set of two adjectives are not coordinate, do not use a comma.

• Correct: The juggler used three yellow balls.
• Incorrect: The juggler used yellow three balls.

In the above examples, three and yellow are not considered of equal importance grammatically, so the set of adjectives is not coordinate. Only three yellow balls is correct. Since the set of adjectives is not coordinate, a comma is not used to separate them.

Coordinate adjectives are especially tricky for English-language learners, because proper usage is determined more by context than by hard and fast rules. Refer to the Parts of Speech section for more information about coordinate adjectives.

Commas for three or more adjectives in a series

Use commas to separate three or more adjectives in a series. The coordinating conjunction and may be placed before the final adjective.

• Example: The hot, humid, rainy day seemed to drag on forever.
• Example: The hot, humid, and rainy day seemed to drag on forever.

Both examples above are grammatically correct. Generally, the use of and before the final adjective is more common in both informal and academic writing.

Use Commas After Introductions

Sometimes, your writing may include an introductory word, clause, or phrase at the beginning of a sentence to provide additional context. A comma should generally be used to separate an introduction from the independent clause of a sentence.

Introductory words

Use a comma after introductory words like however, nonetheless, or furthermore.

• Example: Film production was halted due to the pandemic. Nonetheless, the actors continued to rehearse their roles.

Introductory clauses

If a dependent clause comes before an independent clause in a sentence, use a comma to separate the clauses. Do not use a comma if the independent clause comes first in the sentence.

• Example: Once the snowstorm ended, we had an epic snowball fight.
• Example: We had an epic snowball fight once the snowstorm ended.

Introductory phrases

Use a comma to separate a long introductory phrase from the independent clause in a sentence.

• Example: Before the biggest snowstorm of the year, we made sure to gather firewood.

If an introductory phrase is only a few words long, comma usage is optional.

• Example: Before the snowstorm, we made sure to gather firewood.
• Example: Before the snowstorm we made sure to gather firewood.

How to Use Commas to Separate Unnecessary Words

Unnecessary words

Sentences sometimes contain words that provide additional context or information but are not strictly necessary for the sentence to function.

For a word to be considered unnecessary, it must:

• Not serve a critical role in the grammar of the sentence
• Not serve a critical role in the meaning of the sentence

Use a comma to separate unnecessary words from the rest of a sentence.

Here is an example of a sentence with an unnecessary word.

• Example: She stayed home.
• Example: She, however, stayed home.

In the examples above, She stayed home functions as a complete sentence, so the word however is not grammatically necessary. Both examples above convey the same information, so however is not critical to the meaning of the sentence either.

Words that are grammatically necessary

Here is an example of a sentence where a word is grammatically necessary.

• Correct: However hungry the dog may be, don’t feed him your hamburger.
• Incorrect: However, hungry the dog may be, don’t feed him your hamburger.
• Incorrect: Hungry the dog may be, don’t feed him your hamburger.

In the examples above, the word However is necessary for the grammar of the sentence to work. The word However should not be separated by a comma.

Words that are necessary for meaning

Here is an example where a word is necessary for the meaning of a sentence. Imagine that you have two sisters, Catherine and Julie.

• Example: Your sister came to visit you.
• Example: Your sister Catherine came to visit you.

In the above example, the word Catherine is not separated by commas because it is critical to the meaning of the sentence. Without the word Catherine, it is not clear which sister came to visit.

Now imagine that Catherine is your only sister.

• Example: Your sister came to visit you.
• Example: Your sister, Catherine, came to visit you.

In the above example, the word Catherine is separated by commas because it is not critical to the meaning of the sentence. There is only one person that your sister could be referring to.

Other Uses of Commas to Consider

Numbers

Use a comma to separate the thousands place in a numeral.

• Example: 1,000
• Example: 30,000
• Example: 999,999,999

Remember that different style guides have different rules on when to use a numeral and when to use the word form of a number (in other words, 10 instead of ten).

Place-names

Use commas to separate place-name elements from each other. A place-name element is a location name like a street address, city, state, country, or campus.

• Example: I visited my friend in Atlanta, Georgia, a few days ago.
• Example: I visited my friend in Paris, France, a few years ago.
• Example: Pittsburgh, PA, is home to some of the nation’s largest steel mills.
• Example: I presented at a conference at the University of South Florida, Tampa, on the topic of sustainability.

When multiple place-names are listed together in a sentence, a comma should come after the last place-name element, unless it is the last word in the sentence.

• Example: I went to the campus bookstore at 2727 Mansfield Blvd., Wesley Chapel, Florida, to purchase textbooks for my class.

Do not use a comma if there is only one place-name.

• I visited my friend in Atlanta a few days ago.
• I presented at a conference at the University of South Florida on the topic of sustainability.
• I went to the campus bookstore at 2727 Mansfield Blvd. to purchase textbooks for my class.

No matter which style guide you are following, the numbers in a place-name should be represented by numerals. 

Dates

Use a comma for dates expressed in month-day-year format. When this date format is used in a sentence, a comma should come after the year, unless it is the last word in the sentence.

• Example: Comic book writer Stan Lee was born on December 28, 1922, and died on November 12, 2018.

Do not use a comma for dates expressed in day-month-year format.

• Example: Comic book writer Stan Lee was born on 28 December 1922 and died on 12 November 2018.

When using a month and a year in a sentence or a season and a year in a sentence, do not separate the elements of the date with a comma.

• The so-called "Y2K bug” that was supposed to cripple computer systems on January 2000 was vastly overblown.
• I graduated at the end of spring 2022.

No matter which style guide you are following, the numbers in a date should be represented by numerals.

Separating signal phrases from sentence quotes

A signal phrase identifies who is speaking when a quote is used in a sentence. Use commas to separate signal phrases from their quotes.

If a signal phrase comes after a quote, place the comma before the end quotation mark. 

• Example: “There will be an earthquake along the Santa Barbara fault within the next ten years,” according to Morelli.

If a signal phrase comes before a quote, place the comma before the first quotation mark.

• Example: According to Morelli, “There will be an earthquake along the Santa Barbara fault within the next ten years.”

If a signal phrase interrupts a single quoted sentence, follow both of the rules above.

• Example: “There will be an earthquake,” according to Morelli, “along the Santa Barbara fault line within the next ten years.”