Learning when to and when not to use commas is a difficult and sometimes overwhelming task for many writers, but it is absolutely essential because commas are signals that tell readers which words need to be separated from each other or which words are not essential to the basic grammar of a sentence. This is just a quick reference guide for the most common uses and misuses of the comma.
Here are a few of the most common places to use commas:
- To separate items in a list.
We bought eggs, milk, and bread.
- To separate introductory words and phrases from the main clause.
Laughing, he stood up.
Laughing at the stupidity of it, he stood up.
With the laughter getting louder, we realized he had stood up.
- To separate nonessential words and phrases, as well as dependant clauses, from the main clause. In this case, the sentence should still make sense if you remove the nonessential words and phrases.
The cat, which had just eaten the mouse, curled up to go to sleep.
The cat was sleepy after eating the mouse, which wasn’t surprising. [Remember that nonessential doesn’t mean unimportant, just that it’s not part of the main clause or that it is not necessary to identify the subject.]
- Before coordinate conjunctions (for, and, nor, but, or, yet, so) that link independent clauses.
The cat was tired, so it curled up and went to sleep.
Here are a few of the most common mistakes writers make when using commas:
- Using commas to connect two sentences together.
The cat was tired, it curled up and went to sleep. [This creates a comma splice error, a type of run on sentence.]
- Using commas in front of dependent words (subordinating conjunctions) like because, when, although, if, until, unless. [For a more complete list of dependent words, see 38c of the Keys for Writers text.]
The cat was tired, because it had just eaten the mouse.
- Using commas after such as, like, or although.
I like pastas although, I also like many meat dishes.
- Using commas after but and and (coordinating conjunctions).
I like pastas but, I also like many meat dishes.
Comma Do’s and Don’t’s
Understanding when and when not to use commas can be really confusing; however, using commas correctly in our writing helps to take our writing to a more advanced level. It certainly doesn’t “make” the essay, but correct comma use definitely puts the finishing touches on and written work.
This list of comma do’s and don’t’s will help you to learn when you should use commas and when you should leave them out. Don’t give up; practice makes perfect! Review this information along with the information on commas in your Keys for Writers text (Section 47), and you’ll be a comma pro in no time!
- Use a comma after an introductory word or word group.
Finally, the car swerved to the left, avoiding the people on the sidewalk.
Until she noticed the handprints, the preschool teacher thought finger paints were a great idea.
- Use a comma between items in a series – even between the last two items.
Her favorite foods are pizza, lasagna, and spaghetti.
- Use a comma in front of a coordinating conjunction (for, and, nor, but, or, yet, so) when that conjunction joins two independent clauses (main sentences).
His head injury was severe, so the doctors began to worry.
- Use a comma to set off or separate nonessential words or phrases (words or phrases that aren’t necessary to identify the subject) in a sentence from the main clause of the sentence. Remember that nonessential doesn’t mean unimportant, just that it’s not part of the main clause or necessary to identify the subject.
Mr. Franks, my neighbor, wants to plant a garden. (nonessential phrase)
The contest winner, who lost three years in a row, is an example of perseverance. (nonessential adjective clause)
Some people, by their faith in human nature, bring out the best in others. (nonessential adjective phrase)
One activist, the respected W.E.B.Dubois, worked tirelessly for racial equality. (nonessential appositive phrase)
- Use a comma with transitional expressions, parenthetical expressions, contrasting elements, and absolute phrases.
Mr. Wilson, for example, was unable to cope from tireless touring. (transitional expression)
Note: When a transitional expression joins two independent clauses, a semicolon should be used instead of a comma.
Mr. Wilson was unable to cope from his tireless touring; for example, he was on the verge of a breakdown when the tour finally ended.
Technology, so they say, is indeed the wave of the future. (parenthetical expression)
As an actor, Russell Brand is a comedian, not a dramatist. (contrasting element)
The snake slithered through the tall grass, the sunlight shining now and then on its skin. (absolute phrase).
- Use a comma to set off words of a direct address, and yes and no.
John, do you need some water now?
Yes, I will meet you at noon.
- Use a comma to separate a direct quote from the rest of the sentence.
Michaels declared, “When you get to this stage in your life, the rest is history.”
- Use a comma to separate parts of dates and addresses.
On March 4, 1931, she traveled to Canada.
The President of the United States lives at 1600 Pennsylvania Avenue, Washington, D.C.
- Don’t use a comma to connect two main sentences (independent clauses).
I like to reflect on my life, it is a necessary part of personal growth.
I like to reflect on my life. It is a necessary part of personal growth.
Note: Many choices are available for sentence combining. See our review of comma splices below for a variety of methods to join sentences. This information can also be found in Section 39 of your Keys for Writers text.
- Don’t use a comma to separate the subject and verb in an independent clause.
Reflecting on one’s life, is a necessary part of personal growth.
Reflecting on one’s life is a necessary part of personal growth.
- Don’t add a comma before the first or after the last item in a series.
Americans work longer hours than, German, French, or British workers, are expected to work.
Americans work longer hours thanGerman, French, or British workersare expected to work.
- Don’t add a comma after “such as” or “like” or “although.”
I like Italian pastas such as, lasagna, spaghetti, and fettuccini.
I like Italian pastas such as lasagna, spaghetti, and fettuccini.
Women were portrayed in Elizabethan drama although, they were acted by men.
Women were portrayed in Elizabethan drama although they were acted by men.
- Don’t use a comma to separate compound word groups unless they are independent clauses.
His head injuries were so severe that the doctors became worried, and wanted to run more tests.
His head injuries were so severe that the doctors became worried and wanted to run more tests.
- Don’t use a comma to separate essential words and phrases from the main clause of the sentence.
The neighbor, who lives behind me, wants to plant a garden.
The neighbor who lives behind me wants to plant a garden.
Note: The neighbor can’t be indentified without the phrase “who lives behind me,” so this lets us know that this phrase is essential to the sentence and should not have commas around it.
- Don’t use a comma in front of dependent conjunctions (because, although, until, since, when, etc.).
The children need to be tested again, because the first results were unreliable.
The children need to be tested again because the first results were unreliable.
- Don’t use a comma between adjectives that cannot be reversed in order (cumulative adjectives).
Three, thirsty hikers emerged from the mountain.
Three thirsty hikers emerged from the mountain.
Note: Because we cannot reverse the order of these adjectives and have the sentence make sense, we can’t use a comma to separate them. Another tip is to try replacing the comma with “and.” If we can do this and the sentence still makes sense, then we need the comma. If it doesn’t make sense, then we need to remove the comma from the adjectives.
- Don’t use a comma between adjectives and nouns.
A review by a published, writer would be helpful.
A review by a published writer would be helpful.
- Don’t use a comma between adverbs and adjectives.
My house is a delightfully, chaotic environment for children.
My house is a delightfully chaotic environment for children.
- Don’t use a comma after coordinating conjunctions (for, and, nor, but, or, yet, so).
His head injury was severe so, the doctors began to worry.
His head injury was severe, so the doctors began to worry.
One major sentence error that writers often struggle with is the comma splice error. This error is a type of run on sentence because it contains two main ideas or sentences joined incorrectly with only a comma. A comma is used to create pause in a sentence and to separate the main idea from nonessential elements of the sentence. It is not a strong enough piece of punctuation to join two sentences together.
Consider this comma splice:
Joe likes eggs, Melissa likes cereal.
Here we have two sentences or complete ideas that are joined only by a comma, but remember, the comma is not a strong enough punctuation mark to join two complete sentences. Here are some corrected versions:
Joe likes eggs, but Melissa likes cereal. (coordinating conjunction added)
A comma can only be used to join two sentences together when it is helped out by a coordinating conjunction like for, and, nor, but, or, yet, so.
Joe likes eggs; Melissa likes cereal. (semicolon used instead of a comma)
We can replace the comma with the semicolon because the semicolon is a much stronger punctuation mark. It is strong enough by itself to join two sentences together. One caution about the semicolon, though: It should only be used to join sentences that have closely related ideas, and using it too frequently can create choppiness in our writing. Also, the words on each side of the semicolon should create a full sentence (independent clause). If not, the semicolon probably hasn’t been used correctly.
Joe likes eggs; however, Melissa likes cereal. (semicolon with conjunctive adverb)
We can use a conjunctive adverb after the semicolon if we want to more clearly show a relationship between the sentences. Conjunctive adverbs are words like however, nevertheless, consequently, as a result, therefore. Remember that a comma needs to follow the conjunctive adverb, and a semicolon should come before it as in the example above.
Although Joe likes eggs, Melissa likes cereal. (subordinating conjunction at the beginning of sentence, so a comma is acceptable punctuation)
We can use a subordinating conjunction (dependent word) like because, although, as, until, unless, when to join two sentences together. Notice that in the above example we have the dependent word although at the beginning of the sentence. Because of this, the clause “Although Joe likes eggs” becomes a dependent clause, meaning that it cannot stand alone as a complete sentence. What we end up with, then, is one dependent clause and one independent clause (complete sentence). A comma is strong enough to join a dependent and independent clause together, so this comma use is acceptable and does not create a comma splice error.
If we wanted, we could also invert the order of these clauses to come up with this:
Melissa likes cereal although Joe likes eggs.
In this case, the dependent word is in the middle of the sentence, so no comma is needed (remember that commas are usually not needed in front of dependent words).
Sometimes we need to use a relative pronoun like which or that to correct a comma splice. Here is an example:
Joe likes eggs, these are his favorite breakfast food. (comma splice)
Joe likes eggs, which are his favorite breakfast food. (correct)
For more help with correcting comma splice errors, you can review Section 39 of the Keys for Writers text.
ARC Home | Writing a manuscript
Editing Tip: Parenthetical Elements
This tip reviews how to properly use parenthetical elements, clauses and phrases that can help clarify meaning in your scientific writing
A parenthetical element is information that is nonessential to the meaning of a sentence, such as an example, a clarification, or an aside. This type of sentence component may include the following types of clauses and phrases, as long as the information is nonrestrictive:
- Relative clauses, which commonly begin with which, who/whom/whose, where, or when
- Appositives, or nouns or phrases that rename preceding nouns or phrases
- Participial phrases, or verb-based phrases that describe preceding nouns
- Prepositional phrases, or preposition-based phrases that often describe preceding nouns
- Phrases beginning with such as, including, e.g., or i.e.
The supplementary information provided by a parenthetical element is typically enclosed by two commas, parentheses, or dashes, separating the nonessential material from the rest of the text. Although these punctuation marks have a common purpose, the content of their associated text and their level of emphasis of this text may vary.
Commas are frequently used as a slight interruption, as in the following examples:
- The experiment, which was performed in triplicate, yielded significant results. (relative clause)
- The experiment, the first of its kind, yielded significant results. (appositive)
- The experiment, requiring months of planning, yielded significant results. (participial phrase)
- The experiment, after months of planning, yielded significant results. (prepositional phrase)
- The experiment, including three replicates and two controls, yielded significant results. (“including” phrase)
In each of these sentences, the phrase bracketed by commas is the parenthetical element. In other words, the sentence “The experiment yielded significant results” maintains its meaning when the parenthetical element is removed.
Note that in the cases of e.g. and i.e., not only the parenthetical information but also the abbreviation itself should be enclosed by commas, as in “The main limitations of this study, i.e., the small sample size and the lack of a positive control, should be noted.”
Similar to commas, parentheses separate nonessential information from the rest of the sentence. However, this punctuation emphasizes its content slightly more than commas do, as in “The experiment (the first of its kind) yielded significant results.” Moreover, parentheses do not require a specific grammatical connection to the rest of the sentence, as demonstrated below:
- The experiment yielded significant results (p<0.05) after the inhibitor was added. (data)
- The experiment yielded significant results (see Table 1 for more information) after the inhibitor was added. (independent clause)
Note that in these examples, the parenthetical information does not fall into any of the categories of common nonrestrictive elements defined above.
Dashes are typically used to strongly emphasize a parenthetical element, even more than parentheses do, often conveying a sudden change in tone or content. For instance, an appositive is highlighted by dashes in the following sentence: “This result—an unforeseen consequence of our technique—motivated us to modify the protocol.” Although commas or parentheses would also be grammatically correct here, the dashes reinforce the unexpected nature of the finding. However, note that in scientific writing, commas and parentheses are more frequently used than dashes, which are sometimes construed as informal.
Dashes may also be used to set off a parenthetical element containing commas, as in the following example: “The main limitations of this study—including the small sample size, the lack of a positive control, and its regional focus—should be noted.”
How to choose?
The decision of whether to use a pair of commas, parentheses, or dashes is often a judgment call, dependent on the perceived importance of the information being communicated. Generally, as William Safire wittily wrote, “Put in olive-sizing terms, commas are large, parentheses giant and dashes supercolossal.” However, keep in mind that although these punctuation marks can add important clarifying information to a sentence, their overuse can be distracting to the reader, interrupting the flow of ideas. As in many areas of life, moderation is best.
If you have any questions or comments about this editing tip, please write to us at [email protected]. We wish you the best in your writing endeavors!
TagsWriting a manuscriptEditing tipsPunctuationSentence and paragraph structureClarity in writingParentheticalsClausesPhrasesParenthesesAppositives