Commas
Commas are used to mark off parts of a longer sentence to make the meaning clearer. There are four basic uses for the comma:
- Joining two sentences with a linking word
- Adding extra information to a sentence
- Listing items in a series
- Separating addresses and large numbers
If a comma does not match one of these uses it should be removed from your writing.
Joining two sentences with a linking word
A comma can be used to join two sentences, but there must be a linking word (such as, and, so, but, or because) between them.
If there is no linking word you have created a comma splice, which is considered to be bad punctuation.
I came out to Massey to check my timetable, I went to my lecture as well.
I came out to Massey to check my timetable, and I went to my lecture as well.
I tried to get some milk, the dairy had sold out.
I tried to get some milk, but the dairy had sold out.
Note: it is not always necessary to put a comma before some linking words when both sentences are simple and short:
I met my friend and we went to our lecture.
How to spot a comma splice
If you can replace the comma with a full stop, you need to add a linking word or you will have a comma splice. You can also change the comma to a full stop, a semi-colon, or (sometimes) a colon: see the section on other punctuation for alternative solutions to a comma splice error.
Adding extra information to a sentence
Commas are often used to add extra information to a basic sentence.
If you add the extra information at the start or the end of a sentence, you only need one comma:
However, I got some at the supermarket.
In the end, I borrowed some off my flatmate.
The baby went to sleep at dawn, to his parents' relief.
I borrowed some socks off my flatmate, the generous chap.
However, if you add the extra information to the middle of a sentence, you need to put commas on either side:
The baby, who had cried all night, went to sleep at dawn.
My only transport, a brand-new mountain bike, was stolen yesterday.
In this situation, there must be commas on both sides. Note that you could use brackets or dashes instead of commas here: see the section on other punctuation for more. This is especially useful when the extra information has commas within it: you don't want to confuse the reader about where the extra information begins or ends!
How to tell the difference between necessary information and extra information
The extra information in the above examples (in bold) cannot stand alone as a sentence. If you remove it, however, the rest of the sentence can stand alone:
Who had cried all night.
The baby went to sleep at dawn.
Listing items in a series
Commas are used between items in a list:
I am studying German, English, Philosophy, and History.
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Note: If the distinct items are particularly long or there are commas within one item it is acceptable to use semi-colons instead of commas to separate them:
There are many things on my to-do list. I need to run down to the bank to cash my cheque before 4:30pm; pick up my clothes from the drycleaner, unless they haven't finished cleaning them yet; and take the car to a carwash.
Note: Style guides differ on whether there needs to be a comma before the final item in a list:
I am studying German, English, Philosophy, and History.
vs.
I am studying German, English, Philosophy and History.
This final comma is called the Serial Comma or Oxford Comma. If you've been asked to use a particular style guide, make sure you know and follow the rules regarding this type of punctuation. For example, the serial comma should be included if you are following APA Style.
If you have not been asked to follow a particular style guide, we recommend that you choose one approach and remain consistent throughout your writing.
Separating addresses and large numbers
Commas are used when writing addresses or large numbers to avoid confusion:
235B Ponsonby Road North, Auckland
NZ$3,000,000