Commas can be a particularly tricky punctuation mark. There are some cases where you know you should use a comma – such as when separating items in a list – but there are other times when you might be unsure whether or not a comma is needed.
While there’s some degree of flexibility in how commas are used, it’s important to have a clear grasp of the rules.
Seven Places Where You SHOULD Use Commas
Rule #1: Use Commas to Separate Items in a List
This probably the first use of commas you learned in school: separating items in a list of three or more things.
Here’s an example:
The cake mix requires flour, sugar, eggs, and butter.
Note that some style guides would not add the comma after the word “eggs”. For more on this, see Rule #8.
Rule #2: Use a Comma After an Introductory Word or Phrase
When a word or phrase forms an introduction to a sentence, you should follow it with a comma, as recommended by Purdue OWL.
Here are some examples:
However, she didn’t love him back.
On the other hand, it might be best to wait until next week.
Rule #3: Use a Comma Before a Quotation
You should always put a comma immediately before a quotation:
He said, “It’s warm today.”
John Smith told us, “You can’t come in after ten o’clock.”
Rule #4: Use a Comma to Separate a Dependent Clause That Comes BEFORE the Independent Clause
A dependent clause, or subordinate clause, is one that can’t stand alone as a whole sentence. It should be separated from the independent clause that follows it using a comma:
If you can’t make it, please call me.
After the race, John was exhausted.
However, it’s normally not necessary to use a comma if the independent clause comes first:
Please call me if you can’t make it.
John was exhausted after the race.
For more on this, plus an example of an instance where a comma is required after the independent clause, take a look at Subordinate Clauses and Commas.
Rule #5: Use a Comma to Join Two Long Independent Clauses
Normally, you should put a comma between two complete sentences that are joined with a coordinating conjunction (and, or, but, for, nor, so, yet) that creates a single sentence with two independent clauses:
Sue didn’t know whether she had enough money in her account to pay for the groceries, so she went to an ATM to check her balance.
John was determined to get the unicorn slime his daughter wanted, but all the shops had sold out.
You don’t need a comma if both the independent clauses are relatively short and similar in meaning:
Sue went to the shops and John went home.
Rule #6: Use Commas to Set Off an Nonessential Element within a Sentence
Sometimes, you might want to include extra information within a sentence that isn’t essential to its meaning. You should set this information off using a comma before and a comma after it:
John went for a jog, which took half an hour, before having a long hot shower.
Writing a book, if I haven’t put you off already, is one of the most rewarding things you can do.
The sections in bold could be removed from the sentences completely and it would still make perfectly good sense. You could also use dashes in this context:
John went for a jog – which took half an hour – before having a long hot shower.
Dashes are useful if you want to imply a longer pause, or draw more attention to the nonessential element of the sentence. They’re also useful if you have several other commas in the sentence, to help avoid confusion.
Rule #7: Use Commas to Separate Coordinate Adjectives
When you’re describing something with two or more adjectives, you can use a comma between them if those adjectives are coordinating. (They’re coordinating if you could place “and” between them.) You shouldn’t put a comma after the final adjective.
He’s a cheerful, kind boy.
A comma is used here, because it would also make sense to say, “He’s a cheerful and kind boy”.
There’s a blue bath towel on your bed.
Here, “bath” is acting as an adjective to modify “towel”, but it’s not coordinate with “blue”. It wouldn’t make sense to say, “There’s a blue and bath towel,” so no comma is used.
For more on coordinate and non-coordinate adjectives, check out this post.
One Place When You CAN Use a Comma
While commas are normally either required or not required, there’s one key instance when you can choose whether or not to use a comma – and either option is equally correct.
Rule #8: If You Use a Serial Comma, Use it Consistently
A list of items can be punctuated like this:
We need bread, milk, cheese, and eggs.
Or like this:
We need bread, milk, cheese and eggs.
In the first case, the “serial comma” or “Oxford comma” is used after the penultimate item in the list. In the second case, that comma is omitted.
Some writers have very strong feelings for and against the serial comma. In general, it’s more commonly used in American English than in British English, but you’ll find that opinions vary on both sides of the Atlantic.
Ultimately, it’s up to you (and your editor!) whether or not you use it. The only rule here is to be consistent throughout your piece of writing.
Two Places Where You Shouldn’t Use Commas
Sometimes, writers end up inserting unnecessary commas or using commas incorrectly. Here are two common issues to watch out for in your writing.
Rule #9: Don’t Use a Comma Between Two Independent Clauses (Without a Conjuction)
If you have two independent clauses, you can’t just use a comma to join them. You can use a semi-colon, or you can use a conjunction plus a comma.
Incorrect: There were no clouds in the sky, I went for a jog.
Correct: There were no clouds in the sky; I went for a jog.
Correct: There were no clouds in the sky, so I went for a jog.
The incorrect version is called a “comma splice”.
Rule #10: Don’t Separate a Compound Subject or Compound Object With Commas
If you have a compound subject or a compound object in a sentence that consists of two nouns, you shouldn’t separate the parts of it using commas.
Incorrect: The rain poured down on John, and Sue.
Correct: The rain poured down on John and Sue.
Incorrect: The rain, and the wind battered the house.
Correct: The rain and the wind battered the house.
I hope this helps you make more sense of commas. They’re a tricky punctuation mark because they’re used in so many different contexts. Many writers do struggle with them, so don’t feel bad if you find them hard to get to grips with.
If you’re finding commas particularly tricky, though, you might want to use an app like ProWritingAid (reviewed here) to help check your writing. As well as helping you ensure your writing is correct, this will make you more aware of when you’re not using commas correctly.