When to Use a Comma: 10 Rules and Examples
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
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.
For
example:
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.
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.
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.
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.
For
instance:
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.
Note: Bear in mind that the above is based on American English. In British English, a comma is not used before the 'and' in a series, e.g. in Rule # 1 above, the example would read: The cake
mix requires flour, sugar, eggs and butter.
