The comma [ , ] is one of the most frequently used forms of punctuation. It has many different purposes and rules for usage. As such, many people find mastering comma use a challenging task.
1. Use a comma to separate the elements in a series (three or more things), including the last two
- He hit the ball, dropped the bat, and ran to first base.
You may have learned that the comma before the and is unnecessary, which is fine if your list contains only shorts items. However, there are situations in which, if you don’t use this comma (especially when the list is complex or lengthy), these last two items in the list will try to glom together, and the reader may have trouble distinguishing whether they are one item or two. Using a comma between all the items in a series, including the last two, avoids this problem. This last comma—the one between the word and and the preceding word—is often called the serial comma or the Oxford comma. Consider the following examples:
- Mohamad, Rashid, and I are studying.
- Mohamad, Rashid and I are studying.
Although both sentences are correct, they mean different things. The first sentence means that all of them are studying. In the second one, the speaker is letting Mohamad know that he and Rashid are studying.
2. Use a comma + a conjunction (and, but, for, nor, yet, or, so) to connect two independent clauses
- He hit the ball well, but he ran toward third base.
One of the most frequent errors in comma usage is the placement of a comma after a coordinating conjunction. We cannot say that the comma will always come before the conjunction and never after, but it would be a rare event, indeed, that we need to follow a coordinating conjunction with a comma. When speaking, we do sometimes pause after the little conjunction, but there is seldom a good reason to put a comma there.
For additional information, see our section on coordinating conjunctions. See the note below regarding the use of a comma between two independent clauses when the second independent clause begins with a parenthetical element or adverbial clause.
3. Use a comma to set off introductory elements
- Running toward third base, she suddenly realized how silly she looked.
It is permissible to omit the comma after a brief introductory element if the omission does not result in confusion or hesitancy in reading. If there is ever any doubt, use the comma, as it is always correct.
4. Use a comma to set off parenthetical elements
- The Champlain Bridge, which spans the St. Lawrence River, is falling down.
By parenthetical element, we mean a part of a sentence that can be removed without changing the essential meaning of that sentence. The parenthetical element is sometimes called added information or appositive. This is the most difficult rule in punctuation because it is sometimes unclear what is added or parenthetical and what is essential to the meaning of a sentence.
Appositives are almost always treated as parenthetical elements. Consider the appositives in the sentence below:
- Calhoun’s ambition, to become a goalie in professional soccer, is within his reach.
- Eleanor, his wife of thirty years, suddenly decided to open her own business.
Sometimes the appositive and the word it identifies are so closely related that the comma can be omitted, as in “His wife Eleanor suddenly decided to open her own business.” We could argue that the name Eleanor is not essential to the meaning of the sentence (assuming he has only one wife), and that would suggest that we can put commas both before and after the name (and that would, indeed, be correct). But his wife and Eleanor are so close that we can regard the entire phrase as one unit and leave out the commas. With the phrase turned around, however, we have a more definite parenthetical element and the commas are necessary: “Eleanor, his wife, suddenly decided to open her own business.” Consider, also, the difference between “College President Ira Rubenzahl voted to rescind the withdrawal policy” (in which we need the name Ira Rubenzahl or the sentence doesn’t make sense) and “Ira Rubenzahl, the college president, voted to rescind the withdrawal policy” (in which the sentence makes sense without his title, the appositive, and we treat the appositive as a parenthetical element, with a pair of commas).
As pointed out above (Rule Number 3), an adverbial clause that begins a sentence is set off with a comma:
- Although Queasybreath had spent several years in Antarctica, he still bundled up warmly in the brisk autumns of Winnipeg.
- Because Tashonda had learned to study by herself, she was able to pass the entrance exam.
When an adverbial clause comes later on in the sentence, however, the writer must determine if the clause is essential to the meaning of the sentence or not. A because clause can be particularly troublesome in this regard. In most sentences, a because clause is essential to the meaning of the sentence, and it will not be set off with a comma:
- The families had to leave their homes because the drought conditions had ruined them.
Sometimes, though, the because clause must be set off with a comma to avoid misreading:
- I knew that Prime Minister Campbell was in trouble because my sister-in-law worked in Parliament and called me with the news.
Without that comma, the sentence says that Prime Minister Campbell’s trouble is the fault of my sister-in-law. Campbell’s troubles did not emerge because my sister-in-law worked in Parliament, so we set off that clause to make the meaning clearly parenthetical.
When a parenthetical element — an interjection, adverbial modifier, or even an adverbial clause — follows a coordinating conjunction used to connect two independent clauses, we do not put a comma in front of the parenthetical element.
- The Blue Jays were leading the league at the end of May, but of course, they always do well in the spring. [No comma after but]
- The Red Blacks spent much of the season at the bottom of the league, and even though they picked up several promising rookies, they expect to be there again next year. [No comma after and]
When both a city’s name and that city’s state or country’s name are mentioned together, the state or country’s name is treated as a parenthetical element.
- We visited North Bay, Ontario, last summer.
- Paris, France, is sometimes called “The City of Lights.”
When the state becomes a possessive form, this rule is no longer followed:
- Kemptville, Ontario’s investment in the insurance industry is well known. (no comma after Ontario)
Also, when the state or country’s name becomes part of a compound structure, the second comma is dropped:
- Heublein, a Hartford, Connecticut-based company, is moving to another state. (no comma after Connecticut)
An absolute phrase (a phrase that consists of a noun or pronoun and a participle and modifies the whole sentence) is always treated as a parenthetical element, as is an interjection. An addressed person’s name is also always parenthetical. Be sure, however, that the name is that of someone actually being spoken to.
- Their years of training now forgotten, the soldiers broke ranks.
- I’m telling you, Juanita, I couldn’t be more surprised. (I told Juanita I couldn’t be more surprised. [No commas])
- Yes, it is always a matter, of course, of preparation and attitude.
5. Use a comma to separate coordinate adjectives
If you can put an and or a but between adjectives in a sentence, a comma will probably belong there (if you decide to omit the conjunction). For instance, you could say, “He is a tall and distinguished fellow” or “I live in a very old and run-down house.” So without the and you would write, “I live in a very old, run-down house.” But you would probably not say, “I live in a little and purple house,” so commas would not appear between little and purple.
6. Use a comma to set off quoted elements
Because we don’t use quoted material all the time when writing, this is a difficult rule to remember in comma usage. Generally, use a comma to separate quoted material from the rest of the sentence that explains or introduces the quotation:
- Summing up this argument, Peter Coveney writes, “The purpose and strength of the romantic image of the child had been to establish a relation between childhood and adult consciousness.”
If an attribution of a quoted element comes in the middle of the quotation, two commas will be required. But be careful not to create a comma splice in so doing.
- “The question is,” said Alice, “whether you can make words mean so many things.”
Be careful not to use commas to set off quoted elements introduced by the word that or quoted elements that are embedded in a larger structure:
- Peter Coveney writes that “[t]he purpose and strength of . . .”
- We often say “Sorry” when we don’t really mean it.
And, instead of a comma, use a colon to set off explanatory or introductory language from a quoted element that is either very formal or long (especially if it’s longer than one sentence):
- Peter Coveney had this to say about the nineteenth-century’s use of children in fiction: “The purpose and strength of . . .”
7. Use commas to set off phrases that express contrast
- Some say the world will end in ice, not fire.
- It was her money, not her charm or personality, that first attracted him.
- The puppies were cute, but very messy.
(Some writers will leave out the comma that sets off a contrasting phrase beginning with but.)
8. Use a comma to avoid confusion
This is often a matter of consistently applying Rule Number 3.
- Outside the lawn was cluttered with hundreds of broken branches.
- Outside, the lawn was cluttered with hundreds of broken branches.;
9. Use a comma for typographical reasons
Between a city and a province (Montreal, Quebec], a date and the year [June 15, 1997], a name and a title when the title comes after the name [Bob Downey, Professor of English], between a name and suffix [Bob Downey, Jr.], in long numbers [5,456,783 and $14,682], etc.
10. Use a comma to set off concluding elements
It is important to set off concluding elements to avoid confusion. Consider the following examples.
- Let’s cook Grandpa!
- Let’s cook, Grandpa!
In the first example, it sounds as though the person speaking plans on cooking her grandfather. With the comma (in example 2), it is clear that the speaker is inviting her grandfather to cook with her.
Use Commas with Caution
As you can see, there are many reasons for using commas, and we haven’t listed them all. Yet the biggest problem that most students have with commas is their overuse. Remember that a pause in reading is not always a reliable reason to use a comma. Try not to use a comma unless you can apply a specific rule from this page to do so.