When you have a list of nouns in a sentence, you should put a comma after each noun except the last one, and sometimes the second to last one, with a space after each comma but not before. In the United States it is common to put a comma after every noun except the last one, as in the following sentences:
The last comma (the one before the word 'and' in bold above), is known as the Oxford comma or the serial comma and isn't needed but is often used anyway.
In the United Kingdom it is more common to leave out the Oxford comma (before the word "and") as in the following sentences:
Sometimes that last comma is needed so it is clear what you are talking about. Look at the following sentence:
In the above sentence it is not clear if he called two people or four people. Are the principal and the police chief his parents? If they aren't, you need the Oxford comma, as below:
Now it is clear that he called four people, and that his parents are not the principal and the police chief.
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