Generally speaking, commas are used to connect two or more elements in a sentence, but the way in which they do this varies widely, depending on what these elements are and how they are arranged in the sentence.
Joining independent clauses
When we link two or more closely related independent clauses together to form a compound sentence, we usually do so by adding a comma followed by a coordinating conjunction (and, but, or, so, for, nor, or yet). For example:
- “The family moved into the new house, and the neighbors welcomed them warmly.”
- “She wanted to play tennis, but he wanted to play basketball.”
- “We can go to the movies tonight, or we can just stay home.”
We can also use a semicolon instead of a comma to join independent clauses; the semicolon can either stand on its own or be used with a conjunctive adverb. (Note that we must always use a comma with a conjunctive adverb, as it is acting as an introductory element.) For example:
- “We can go to the movies tonight; however, I would rather just stay home.”
- “She wanted to play tennis; he wanted to play basketball.”
A comma splice occurs when we try to join two independent clauses using only a comma on its own. A comma alone, however, is not strong enough to separate two independent clauses. For example:
Luckily, the mistake is easy to correct using any of the methods for forming compound sentences that we have already described. For example:
Joining items in a list
In addition to joining independent clauses, coordinating conjunctions can also join two people or things in a sentence. When we are listing more than two people or things in a row, we separate each item with a comma, using a coordinating conjunction between the last and second-to-last item. For example:
- “I enjoy swimming, hiking, and riding my bicycle.”
- “Either Tom, Bill, Jeff or Mike will lead the seminar.”
The Oxford Comma
Notice that the first example has a comma before the coordinating conjunction (and), while the second example does not. When we use a comma before this coordinating conjunction, it is called a serial comma or Oxford comma.
There are strong opposing opinions over whether this comma should ever be used. Neither side is right or wrong, though; it’s simply a stylistic preference. Therefore, all of the following sentences could be considered correct:
- “I like apples, bananas, pears, and figs.
- “I like apples, bananas, pears and figs.
- “She’s smart, beautiful, and witty.”
- “She’s smart, beautiful and witty.”
- “We have always wanted to buy a boat, sell everything, and set sail.”
- “We have always wanted to buy a boat, sell everything and set sail.”
It should be noted that certain varieties of English use the serial comma more than others. For example, most American English style guides recommend its use. On the other hand, the majority of British English style guides recommend against it, with the most important exception being the Oxford Style Manual (from which the “Oxford comma” received its name).
Joining coordinate adjectives
When we use multiple adjectives to describe the same noun, we may or may not use commas between them, depending on how the adjectives function together.
In general, we do use commas between adjectives that describe the noun independently from one another. These are called coordinate adjectives. For example:
- “I bought a heavy, long table.”
Each of the above adjectives separately describes the noun table. One way that we can check if adjectives are coordinate is by trying to switch around the order and see if the sentence still makes sense. For example:
- “I bought a long, heavy table.”
The sentence still sounds correct, so we know that we are using coordinate adjectives and need to use a comma. Another way that we can check is by inserting the word and where the comma would go:
- “I bought a heavy and long table.”
Again, the sentence still sounds correct, so we know we are dealing with coordinate adjectives. Here are a few more examples of sentences using coordinate adjectives:
- “It was a dark, cold, blustery day.”
- “There’s no way I’m staying in an old, dirty, drafty room like this!”
Repetition of adjectives and adverbs
When an adjective or adverb is repeated to add emphasis to a description, we use a comma between the repeated words. For example:
- “Your mother and I are very, very proud of you!”
- “It is just a sad, sad occasion.”
Adjectives that build on each other to create a complete description, rather than functioning independently, are called cumulative adjectives, and we don’t separate them with commas:
- “I bought a black wooden table.”
Black describes wooden table (not just table alone), and so this sentence would sound strange if rearranged, like this:
- “I bought a wooden black table.”
We can also try inserting and, with the same result:
- “I bought a black and wooden table.”
The sentence doesn’t sound right when rearranged or when and is added, so we know that we are dealing with cumulative adjectives, and we should not separate them with a comma.
We also use commas to link introductory information at the beginning of a sentence. This can be a single word, a phrase (a group of two or more words), or an entire dependent clause. For example:
- “Strangely, he wrote to Michelle but not to me.”
- “In a way, they are both right.”
- “When I was traveling in Croatia, I met a lot of interesting people.”
There are some cases in which the introductory element is brief enough or so closely related to the independent clause that the comma is often omitted. The second example, for instance, has an introductory phrase with clear meaning and relation to the sentence, so we could probably leave the comma out, as in:
- “In a way they are both right.”
However, we can only do this if it does not make the sentence confusing or difficult to read. We can see that confusion arises if we omit the commas in the other two examples:
- “Strangely he wrote to Michelle but not to me.”
- “When I was traveling in Croatia I met a lot of interesting people.”
By leaving out the comma in the first sentence, it looks as though strangely is just modifying the verb wrote, rather than the entire sentence. In the second example, the sentence runs together and it becomes a bit harder to read without the comma.
Because we must exercise such care when deciding whether to include or omit a comma after an introductory element, the most straightforward guidance is to always include it, as it is always correct to do so.
Afterthoughts and closing comments
Much in the way we place a comma after an introductory element, we usually use a comma before a final or closing thought at the end of a sentence. For example:
- “I never said that I would have the project finished by that date, just to be clear.”
- “I find such behavior appalling, frankly.”
- “I would like to have a bit more, please.”
- “She works for a financial firm, but she is a remarkably accomplished theater director, too.”
Parenthetical information is not considered critical to the meaning of the sentence as a whole—if we removed it completely, the sentence would still make as much sense as before. Parenthetical elements can function as introductory or closing elements, as we have seen, but they can also appear anywhere in a sentence. If they appear in the middle, they are separated by two commas.
An absolute phrase (sometimes known as an absolute construction) is usually made up of a noun or pronoun and a participle, along with any modifying information. It is always considered a parenthetical element, so it is always set apart by one or two commas. (Note that we can sometimes use em dashes instead of commas to highlight the absolute phrase more dramatically.)
We usually use absolute phrases at the beginning of a sentence to introduce additional information, or at the end of a sentence to provide a final comment on the sentence as a whole. However, it’s also possible to use an absolute phrase in the middle of a sentence to highlight or put extra emphasis on the extra information. For example:
- “The test finished, Jason heaved a sigh of relief.”
- “She walked out the door, her head turning for a last look at home.”
- “I hope—God willing—to get into Harvard next year.”
Appositives most commonly provide parenthetical (nonessential) information about the noun they describe. These are known as non-restrictive appositives, and they are indicated by the commas surrounding them. Remember, if the appositive appears in the middle of the sentence, it is surrounded by two commas; if it appears at the beginning or end, it is followed or preceded by just one. For example:
- “The heir, Prince William, is adored by many.” (The proper noun Prince William provides a name for the noun heir.)
- “The office, an old colonial building, badly needed repairs.” (The appositive phrase an old colonial building describes the noun office.)
- “A true classic, this book inspired a generation of young readers.” (The appositive phrase a true classic describes the noun book.)
Keep in mind that appositives can be essential to the meaning of a sentence. These are known as restrictive appositives, and they are not set apart by commas. Most commonly, these are proper nouns that rename a common noun, though appositive noun phrases can occasionally be restrictive, too. For instance:
- “The popular restaurant Joe’s Place gets thousands of diners a day.”
- “Jeremy Jones the professor has gained much more praise than Jeremy Jones the novelist ever did.”
Relative clauses (also known as adjective clauses or adjectival clauses) are dependent clauses that provide descriptive information about a noun or noun phrase. Relative clauses are introduced by either a relative pronoun (who, whom, whose, which, and that) or a relative adverb (where, when, and why).
Like appositives, the information relative clauses provide can either be essential (restrictive) or nonessential (non-restrictive) to the completeness of the sentence; only non-restrictive relative clauses are set apart by commas. For example:
- “The woman down the street, whose children are the same age as ours, invited us over for dinner next week.”
- “Samantha, whom I’ve asked to be my bridesmaid, is getting married next year.”
- “The movie, which is my favorite comedy of all time, is on TV tomorrow night.”
- “I’m going on a date with Paul, who went to high school with my brother.”
Restrictive relative clauses
Restrictive relative clauses, like restrictive appositives, introduce information that must be included for the sentence to make sense, and they are not set apart by commas:
- “I saw the guy who delivers my mail in town yesterday.”
- “This was the river where we learned to swim.”
- “There are a few different things that we’ve been working on lately.”
- “Do you know the reason why the sky is blue?”
An interjection, also known as an exclamation, is a word, phrase, or sound used to convey an emotion such as surprise, excitement, happiness, or anger. Interjections can stand alone as minor sentences, punctuated with a period, exclamation point, or question mark. However, since interjections are not “proper” sentences, some writers prefer to attach them to a complete sentence with a comma. (Ultimately, it is up to preference.)
- “Ooh, that’s a beautiful dress.”
- “Brr, it’s freezing in here!”
- “Oh my God, where did you get all this money?”
- “Wow, what a great achievement!”
A noun of address (technically known as a vocative) is the person or thing you are directly addressing. Like interjections, they are grammatically unrelated to the rest of the sentence—they don’t modify or affect any other part of it. Instead, they are used to let the listener or reader know who is being spoken to, or to get that person’s attention. For example:
- “James, I need you to help me with the dishes.”
- “Can I have some money, Mom?”
- “This, class, is the video I was telling you about.”
- “Sorry, Tom, I didn’t see you there.”
- “You in the back, do you have a question?”
Another type of parenthetical element occurs when we introduce information that strongly contrasts with the rest of the sentence. We separate the contrasting information with either one or two commas (depending on its position in the sentence). For example:
- “I told you to buy four cases, not six.”
- “The manager, not employees, will decide when it is appropriate to close the restaurant each night.”
- “It is not the wealthy elite who decide elections, but ordinary citizens like you and me.”
Other parenthetical information
We can also use other phrases or clauses parenthetically to provide supporting information or commentary on the rest of the sentence. For example:
- “Find me something to dig with, such as a shovel or spade, so I can plant these flowers.”
- “However, the rules, if we choose to follow them at all, are not always consistent.”
- “Whatever you decide to do, just leave me out of it!”
Be aware that if the parenthetical element contains internal commas, em dashes might be required instead of commas to provide better clarity in the sentence:
- “Many fundamental aspects of living on one’s own—cooking, cleaning, doing laundry—are things for which many young adults are completely unprepared.” (Em dashes are preferable here, because they clearly set apart the parenthetical information from the rest of the sentence.)
- “Many fundamental aspects of living on one’s own, cooking, cleaning, doing laundry, are things for which many young adults are completely unprepared.” (Incorrect: too many internal commas create a confusing sentence that is hard to read.)
Note that in each of these examples, we could have also used parentheses instead of commas or em dashes to indicate the parenthetical information.
Parenthetical elements in compound sentences
There is one exception to the use of two commas to set apart parenthetical elements that appear in the middle of the sentence. When a parenthetical element occurs directly after a coordinating conjunction that links two independent clauses in a compound sentence, it is not necessary to include the first comma—instead, we only place one comma at the end of the parenthetical information. For example:
- “Our sales have improved over the year, but in my view, we still should remain cautious with our investments.”
- “Sarah has her mind set on moving to London, so try as you might, you’re not going to get her to change her decision.”
It can be helpful, however, to include it when the parenthetical element is quite long or needs to be given more emphasis in the sentence. For instance:
- “The problem seems simple enough at first, but, as you will soon find out for yourselves, there are many complexities that we must consider.”
Note that some writers prefer to always include the first comma, regardless of the length of the parenthetical information. They might write our first two examples like this:
- “Our sales have improved over the year, but, in my view, we still should remain cautious with our investments.”
- “Sarah has her mind set on moving to London, so, try as you might, you’re not going to get her to change her decision.”
However, this is entirely based on personal preference, as both styles are considered correct. Check the style guide of your employer or school, though, as they might have a particular preference for one practice over the other.
Countries and States
Similar to parenthetical information, we set apart the names of countries and states with commas when specifying the location of a particular city. For example:
- “I used to live in Detroit, Michigan, but I moved to Albuquerque, New Mexico, last month.”
- “Paris, Texas, is nothing at all like Paris, France.”
Note that a comma comes both before and after the country or state if it appears in the middle of a sentence. There is often a tendency for writers to leave out the second comma (especially in newspapers and magazines), but this is incorrect—the second comma is almost always necessary.
Omitting the second comma
There are certain instances in which the second comma should not be used after the country or state name. This occurs when the location takes on a possessive form, or is used to create a compound adjective. For instance:
We would also omit the comma if another parenthetical element (such as an appositive or absolute phrase) using em dashes appears directly after the country or state, as in:
Note that if a parenthetical element that directly relates to the country or state is set apart by parentheses, the second comma is still used, but it appears after the parenthetical element. For example:
Listing the year in a date
Similarly to how we set apart the names of countries or states with commas, we also set apart the year when it is included with a date. In American English, the month will typically be listed first, followed by the day’s date, followed by a comma and the year. If the date appears in the middle of the sentence, it is also followed by a comma, as in:
- “It was on July 20, 1969, that American astronauts set foot on the moon for the first time in history.”
- “I was born on January 31, 1988, the same day as my cousin!”
If we also include the day of the week (Monday, Tuesday, Wednesday, etc.), then we put a comma between the day and the month as well as before and after the year. For instance:
- “The senator’s arrest on Friday, August 8th, 1998, marked a turning point in the government’s anti-corruption campaign.”
If the date consists of just the month and the year, then no comma is used:
- “We first opened our doors in August 1981 during a recession, but the business managed to survive.” (correct)
- “We first opened our doors in August, 1981, during a recession, but the business managed to survive.” (incorrect)
Note that in British English, the day’s date usually appears first, followed by the month and the year. (This is sometimes referred to as the international or military format.) We do not use commas in this format. For example:
- “The United Kingdom held a referendum on 23 June 2016 to decide whether or not it should remain within the European Union.”
- “My wife was born in Limerick, Ireland, on 3rd June 1978.”
Using commas with quotations (direct speech)
Direct speech (also known as quoted speech) refers to the direct quotation of something that someone else said. We generally use the past tense of reporting verbs (such as said, asked, told, remarked, etc.) to introduce the quotation.
When used in writing, we separate direct speech with quotation marks. (Note that American English uses double quotation marks ( “ ” ), while British English typically uses single quotation marks ( ‘ ’ ).)
If we are quoting an entire sentence, we set it apart with commas as we would for parenthetical information. For example:
- John said, “I’ll never live in this city again.”
- Mary told him, “I want to have another baby,” which took him by surprise.
- The other day, my daughter asked, “Mommy, why do I have to go to school, but you don’t?”
However, if we are quoting a fragment of speech that is used as an integral part of the overall sentence, then no commas are used. We still use reporting verbs in the past tense, though. For instance:
- John said he feels “really bad” about what happened.
Direct speech before a reporting verb
We can also put direct speech before the reporting verb. Again, we usually use a comma to separate the quoted text from the unquoted text, as in:
- “I can’t wait to see daddy,” my son said.
This rule also holds true if the reporting verb and the person attributed to the quotation appear in the middle of quoted text. Note that if the quoted sentence continues after the reporting verb, then we use a second comma to introduce the second part of the sentence. For example:
- “It will be a long time,” she remarked, “before we’re able to afford a house like this.”
However, if the quoted text after the reporting verb is a new sentence, then we use a period before the rest of the quotation, as in:
- “I hope we get there soon,” said Tom. “I’m tired of being stuck in this car.”
Finally, be aware that if a question mark or exclamation point is used in a quotation that appears before the reporting verb, then we do not use a comma at all:
- “Where are we going?” asked Sally.
- “This is going to be great!” Tom exclaimed. “I’ve always wanted to see Paris.”
End punctuation — American vs. British English
In American English, a period or comma used at the end of direct speech always appears within the quotation marks, as we’ve seen in the examples so far.
In British English, however, if the quotation ends in a period or comma, it is usually placed outside the quotation mark, as in:
Note that if a quoted sentence ends in a question mark or exclamation point that belongs to the quotation, it will appear within the quotation marks. If the question mark or exclamation point belongs to the overall sentence (that is, it isn’t actually part of the quotation), it will appear outside the quotation marks. This is the same in both American and British English. For example:
Other technical uses for commas
There are a few other typographical instances in which a comma should be used, such as in long numbers and for titles that appear after people’s names.
It is standard practice to add one or more commas to long numbers to make them easier to read. (Commas used in this way are technically known as delimiters.)
A number is considered to be “long” if it has more than three digits, and we put commas before each section of three that appears in it (not including decimals, if used). For example:
- “Just over 20,000 students attend the private university.”
- “I can’t believe you won $1,500 in the lottery!”
- “In 2011, the population of England was 53,012,456, while the population of Canada was only 33,476,688!”
- “The final bill was $3,543.21.”
While it is very common for people to leave the comma out of four-digit numbers, it is recommended to always include it, especially in formal or professional writing.
For any long number that is acting as a determiner—directly modifying a noun to indicate how many there are—we always use a comma. However, when a number is being used to indicate the year, a street address, or a page number, we do not use a comma. For instance:
- “Please send the package to 5678 Main Street.”
- “By the year 2014, we had over 2,000 clients across the state.”
- “Refer to the notes on page 1345 in the textbook.”
(Note that if we are listing the amount of pages in a book, then the comma would be used as usual, as in, “This book has 1,345 pages.”)
Other numbering styles
It’s important to note that not all countries use this format for numbers. In many parts of Europe, for instance, commas are used as decimal points, while periods are used as delimiters. Other countries will variously use spaces, apostrophes, or “interpuncts” (a dot that appears halfway between the top and bottom of the line) as delimiters in long numbers.
Here are some examples of how long numbers might appear around the world:
When we are writing in English, we only need to worry about using commas before sets of three numbers and periods as decimal points.
Titles and Certifications
Another technical use of commas is to set apart professional, academic, or royal titles and certifications that come after a person’s name. These function much like parenthetical elements, which we looked at earlier—they do not have an impact on the grammatical structure of the sentence, so they are separated by commas. For example:
- “The board is pleased to announce that John Barry, BA, MA, PhD, will be joining the faculty in September.”
- “Allow me to introduce you to Janet Smith, Director of Surgery.”
- “Please forward the material to my attorney, Barbara Simmons, Esq., for further review.”
Some labels that appear after a name do not generally take a comma anymore; sometimes, including the comma is optional. These include family titles, such as Junior and Senior (usually abbreviated as Jr. and Sr.), and company titles, such as Incorporated or Limited (usually abbreviated as Inc. and Ltd.). For example:
- “Harry Smith Jr. is nothing at all like his father, Harry Smith Sr.”
- “Global Markets Inc. is currently in the process of selling off its subsidiary company, Global Markets Ltd.”
As noted, the comma is still considered optional, so it is not uncommon to see these set apart by commas. If commas are used, they should appear both before and after the title (unless the title ends the sentence), as in:
- “Harry Smith, Jr., is nothing at all like his father, Harry Smith, Sr.”
- “Global Markets, Inc., is currently in the process of selling off its subsidiary company, Global Markets, Ltd.”