The comma is a punctuation mark that indicates a slight pause or a transition of some kind. It serves many different grammatical functions and provides clarity for readers. Commas have many uses, but the situations in which they are used can be broken down into four major categories:
- Put a comma before a coordinating conjunction (for, and, nor, but, or, yet, so) that separates two independent clauses.
- Put a comma after introductory words, phrases, or clauses in a sentence.
- Use commas to set off elements that interrupt or add information in a sentence.
- Use commas to visually separate distinct but related items.
Coordinating Conjunctions
Coordinating conjunctions are conjunctions, or joining words, that are placed between words and phrases of equal importance. Used with coordinating conjunctions, commas allow writers to express how their complete thoughts relate to one another. They also help avoid the choppy, flat style that arises when every thought stands as a separate sentence.
When joining two independent clauses, or clauses that could stand on their own as full sentences, place a comma before the conjunction. If the second independent clause is very short, or if it is an imperative, the comma can be omitted.
Example: He was looking forward to the dance, but he was not sure what he would wear.
Both clauses are independent and could stand on their own as complete sentences. When they are joined in the same sentence, however, they must be connected with a comma and a coordinating conjunction.
Introductory Phrases and Clauses
Put a comma after introductory words, phrases, or clauses that introduce a sentence.
Dependent Clauses
A dependent clause is a group of words that can't stand on its own as a sentence because it does not express a complete thought. Sometimes a dependent clause can be used to introduce a sentence. In this situation, use a comma after the dependent clause.
Example: Because I was running late, I did not have time to eat breakfast.
The first phrase could not stand on its own as a sentence, but when joined to the independent clause by the comma, the sentence is complete.
Note that a dependent clause can come later in the sentence, but in that case, you would not use a comma:
Example: I did not have time to eat breakfast because I was running late.
Only use a comma to separate a dependent and independent clause if the dependent clause is first!
Introductory Words and Phrases
Writers can give readers information that limits or otherwise modifies a main idea that follows. To do so, writers can use introductory words or introductory phrases. These introductory elements can be one word or several. Common introductory elements include transition words and statements about time, place, manner, or condition.
Often, introductory words are also adverbs. Commas are always used to set off certain adverbs, including the following:
- however
- in fact
- therefore
- nevertheless
- moreover
- furthermore
- still
Example: Therefore, it is obvious that we should fund the dam-building project.
If one of these adverbs appears in the middle of a sentence, within one clause, it should be set off by a pair of commas.
Example: The dam, however, will take seven years to construct.
For some adverbs, using a comma is optional. In these situations, say the sentence to yourself. If you think a pause makes your sentence more clear or emphasizes what you want to emphasize, use the comma; otherwise, drop it.
- then
- so
- yet
- instead
- too
- first, second, etc.
Example: First we'll go to the mall. Then we'll go to the pet store.
Example: First, we'll go to the mall. Then, we'll go to the pet store.
Both of those sentence pairs are correct!
Adding Information: Modifiers and Appositives
Modifiers are words or phrases that are added to sentences in order to make their meaning more specific. In order to understand what kind of modifiers require commas, first we have to understand the concept of "restrictiveness."
Nonrestrictive Modifiers
Some modifiers are nonrestrictive, meaning that the sentence would still have essentially the same meaning, topic, and structure without them. They simply add a little extra information.
Example: Katy's new fishbowl is growing some weird algae.
In this sentence, "new" and "weird" are nonrestrictive. The sentence without them would be grammatically correct and have essentially the same meaning. They do not require any commas.
Restrictive Modifiers
Restrictive modifiers, on the other hand, are those whose use is essential to the overall meaning of the sentence. In other words, if you dropped a restrictive modifier from a sentence, the meaning of the sentence would change.
Example: The man who scratched your car left a note on your windshield.
The phrase "who scratched your car" is a restrictive modifier because it explains which man the sentence refers to, and because the sentence would be unclear without it.
Appositives
An appositive is a grammatical construction in which two noun phrases are placed side by side, with one identifying the other.
Example: My sister, Alice Smith, likes jelly beans.
In this sentence, "Alice Smith" is an appositive modifying the noun phrase "my sister." Because the name Alice Smith is just adding information, and the sentence would still have the same basic meaning without it, this is an example of a nonrestrictive appositive. Nonrestrictive appositives do require commas.
On the other hand, a restrictive appositive provides information essential to identifying the noun being described. It limits or clarifies that noun phrase in some crucial way, and the meaning of the sentence would change if the appositive were removed. In English, restrictive appositives are not set off by commas.
Example: He loves the television show Iron Chef.
In this sentence, "Iron Chef" is an appositive modifying the noun phrase "television show." Because the meaning of the sentence would be unclear without "Iron Chef," it is considered restrictive and thus does not require a comma.
Separating Related But Distinct Information
Attribution
Use a comma to set off the attribution (i.e., who said or wrote a quotation) from the quotation itself. If the attribution comes at the end of the quotation, then the comma should go inside the quotation marks, even if the quotation is a complete sentence.
Example: "We really messed up this time," he said.
A pair of commas should be used to set off the attribution when it appears in the middle of the quotation.
Example: "Well," she said, "I think I would prefer to have hamburgers tonight."
Do not replace a question mark or exclamation point in a quotation with a comma.
Example: "Where are we going now?" Eugene asked.
Lists
When there are three or more items in a list, commas should be used between the items.
Example: Buy apples, bananas, and grapefruit at the store.
The final comma, the one before and or or, is known as a serial comma (also called the Oxford or Harvard comma). The serial comma should always be used where it is needed to avoid confusion. Can you see the ambiguity in the example below?
Example: "Thank you for the award. I'd like to thank my parents, Charles Darwin and Lindsay Lohan."
It looks like the speaker's parents are Darwin and Lohan, when in reality, the speaker meant to thank her parents and Charles Darwin and Lindsay Lohan. In this situation, the serial comma needs to be used.
Otherwise, depending on the chosen style guide, it is considered optional. Still, not using the serial comma is relatively uncommon in American English, except in newspapers and magazines.
Accumulation
Another type of relationship between ideas that writers signal to readers with a comma is that of accumulation. Occurring at the end of a sentence, cumulative clauses hook up to a main clause and add further information. They often include additional descriptive details.
Example, "The sun rose slowly over the mountains, warming the faces of the miners in the valley, inviting the jays out from their nests, shimmering in the morning dew, inching the day forward one shadow at a time."
As in this example, accumulative phrases should be separated by commas.
Dates
Commas should also be used when writing dates. There should always be a comma between the day and the year and between the year and the rest of the sentence.
Example: "On December 7, 1941, Japanese planes attacked the U.S. Naval base in Hawaii."
Even when the date is not a dependent clause, as it is in the previous example, the last item in the date should be followed by a comma.
Calling in sick for work, Beth hoped her boss would not suspect anything
The title contains a verb in its introductory phrase, which warrants a comma before the final clause. The comma serves a variety of grammatical functions, including to indicate pauses or set off introductory phrases, as in the title example.