Use commas to separate words and word groups in a simple series of three or more items.
Example: My estate goes to my husband, son, daughter-in-law, and nephew.
Note: When the last comma in a series comes before and or or (after daughter-in-law in the above example), it is known as the Oxford comma. Most newspapers and magazines drop the Oxford comma in a simple series, apparently feeling it's unnecessary. However, omission of the Oxford comma can sometimes lead to misunderstandings.
Example: We had coffee, cheese and crackers and grapes.
Adding a comma after crackers makes it clear that cheese and crackers represents one dish. In cases like this, clarity demands the Oxford comma.
We had coffee, cheese and crackers, and grapes.
Fiction and nonfiction books generally prefer the Oxford comma. Writers must decide Oxford or no Oxford and not switch back and forth, except when omitting the Oxford comma could cause confusion as in the cheese and crackers example.
Hope this helped! :)
