Introduction

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

Elements of Comma Usage

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

EXAMPLES

Example 1: 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 2: 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.

EXAMPLES

Example 1: First we’ll go to the mall. Then we’ll go to the pet store.

Example 2: 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.

EXAMPLES

Example 1: “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 2: “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 3: “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.

EXAMPLES

Example 1: 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 2: “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

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.

Common Comma Mistakes

Comma usage errors fall into two categories: using unnecessary commas and failing to use necessary commas. To avoid making errors when using commas in your writing, you must understand when commas belong (and when they don’t).

Keep the following rules in mind for when to not use commas.

Do not use a comma to separate a subject from its predicate.

• Incorrect: Registering for our fitness programs before September 15, will save you thirty percent of the membership cost.
• Correct: Registering for our fitness programs before September 15 will save you thirty percent of the membership cost.

Do not use a comma to separate a verb from its object, or a preposition from its object.

• Incorrect: I hope to mail to you before Christmas, a current snapshot of my dog Benji.
• Incorrect: She traveled around the world with, a small backpack, a bedroll, a pup tent, and a camera.
• Correct: I hope to mail to you before Christmas a current snapshot of my dog Benji.
• Correct: She traveled around the world with a small backpack, a bedroll, a pup tent, and a camera.

Do not misuse a comma after a coordinating conjunction.

• Incorrect: Sleet fell heavily on the tin roof but, the family was used to the noise and paid it no attention.
• Correct: Sleet fell heavily on the tin roof, but the family was used to the noise and paid it no attention.

Do not use commas to introduce restrictive (i.e., necessary) modifiers.

• Incorrect: The fingers, on his left hand, are bigger than those on his right.
• Correct: The fingers on his left hand are bigger than those on his right.

Do not use a comma before a dependent clause that comes after an independent clause. This is called a disruptive comma.

• Incorrect: The future of print newspapers appears uncertain, due to rising production costs and the increasing popularity of online news sources.
• Incorrect: Some argue that print newspapers will never disappear, because of their many readers.
• Correct: The future of print newspapers appears uncertain due to rising production costs and the increasing popularity of online news sources.
• Correct: Some argue that print newspapers will never disappear because of their many readers.

Do not use a comma after a short introductory prepositional phrase unless you mean to add extra emphasis.

• Incorrect: Before the parade, I want to eat pizza.
• Correct: Before the parade I want to eat pizza.

Do not use a comma between adjectives that work together to modify a noun.

• Incorrect: I like your dancing, cat t-shirt.
• Correct: I like your dancing cat t-shirt.

Do not use a comma to set off quotations that occupy a subordinate position in a sentence, often signaled by the words “that,” “which,” or “because.”

• Incorrect: Participating in a democracy takes a strong stomach because, “it requires a certain relish for confusion,” writes Molly Ivins.
• Correct: Participating in a democracy takes a strong stomach because “it requires a certain relish for confusion,” writes Molly Ivins.

Do not use a comma when naming only a month and a year.

• Incorrect: The next presidential election will take place in November, 2016.
• Correct: The next presidential election will take place in November 2016.

Do not use a comma in street addresses or page numbers, or before a ZIP or other postal code.

• Correct: The table appears on page 1397.
• Correct: The fire occurred at 5509 Avenida Valencia.
• Correct: Write to the program advisor at 645 5th Street, Minerton, Indiana 55555.

Comma Splice Errors

A comma splice occurs when two independent clauses (that is, two complete sentences) are joined only by a comma. In those situations, an acceptable form of punctuation would be a semicolon or a period. For example:

• Incorrect: Every day, millions of children go to daycare with millions of other kids, there is no guarantee that none of them are harboring infectious conditions.
• Incorrect: Many daycares have strict rules about sick children needing to stay away until they are no longer infectious, enforcing those rules can be very difficult.
• Incorrect: Daycare providers often undergo extreme pressure to accept a sick child “just this once,” the parent has no other care options and cannot miss work.

Once you discover where the two independent clauses are “spliced,” there are several ways to separate them. You can make two complete sentences by inserting a period. This is the strongest level of separation. You can use a semicolon between the two clauses if they are of equal importance; this allows your reader to consider the points together. You can use a semicolon with a transition word to indicate a specific relation between the two clauses; however, you should use this sparingly. You can use a coordinating conjunction following the comma, and this also will indicate a relationship. Or, you can add a word to one clause to make it dependent.

For example:

• Correct: Every day, millions of children go to daycare with millions of other kids. There is no guarantee that none of them are harboring infectious conditions.
• Correct: Many daycares have strict rules about sick children needing to stay away until they are no longer infectious; enforcing those rules can be very difficult.
• Correct: Many daycares have strict rules about sick children needing to stay away until they are no longer infectious, but enforcing those rules can be very difficult.
• Correct: Daycare providers often undergo extreme pressure to accept a sick child “just this once” because the parent has no other care options and cannot miss work.

Run-On Errors

While a run-on sentence, also known as a fused sentence, might just seem like a type of sentence that goes on and on without a clear point, the technical grammatical definition of a run-on sentence is one that fuses, or “runs together,” two or more independent clauses without using punctuation to separate them. The independent clauses may not have any punctuation separating them, or they may have a coordinating conjunction between them, but without the comma that needs to accompany it to separate the independent clauses. For example:

• Incorrect: Every day, millions of children go to daycare with millions of other kids there is no guarantee that none of them are harboring infectious conditions.
• Incorrect: Many daycare centers have strict rules about sick children needing to stay away until they are no longer infectious but enforcing those rules can be very difficult.
• Incorrect: Daycare providers often undergo extreme pressure to accept a sick child “just this once” the parent has no other care options and cannot miss work.

If you locate a run-on sentence and find where the two independent clauses “collide,” you can decide how best to separate the clauses. Fixing run-on sentences is very similar to fixing comma splices. You can make two complete sentences by inserting a period. This is the strongest level of separation. You can use a semicolon between the two clauses if they are of equal importance; this allows your reader to consider the points together. You can use a semicolon with a transition word to indicate a specific relation between the two clauses; however, you should use this sparingly. You can use a coordinating conjunction and a comma, and this also will indicate a relationship. Or, you can add a word to one clause to make it dependent.

For example:

• Correct: Every day, millions of children go to daycare with millions of other kids. There is no guarantee that none of them are harboring infectious conditions.
• Correct: Many daycares have strict rules about sick children needing to stay away until they are no longer infectious; however, enforcing those rules can be very difficult.
• Correct: Many daycares have strict rules about sick children needing to stay away until they are no longer infectious, but enforcing those rules can be very difficult.
• Correct: Daycare providers often undergo extreme pressure to accept a sick child “just this once” because the parent has no other care options and cannot miss work.

Summary

Main points from this section include:

• The comma is a punctuation mark that indicates a slight break, pause, or transition.
• Commas are necessary before a coordinating conjunction (for, and, nor, but, or, yet, so) that separates two independent clauses.
• Commas are necessary after introductory words, phrases, or clauses in a sentence.
• Commas are necessary to set off elements that interrupt or add information in a sentence.
• Avoiding unnecessary commas is simply a matter of understanding the rules of correct comma usage.
• A comma splice occurs when two independent clauses are joined only by a comma instead of an acceptable form of punctuation, such as a comma with a coordinating conjunction, a semicolon, or a period.
• A run-on sentence occurs when two or more independent clauses fuse together without punctuation to separate them.

Sources:

“Commas.” By Lumen Learning. Retrieved from https://courses.lumenlearning.com/boundless-writing/chapter/commas/ Licensed under CC-BY-SA 4.0