Frequently Asked Questions
If you ask about the comma before "and" in a series, you are liable to get mixed answers. Rumor has it that the idea to omit the comma started with a 19th Century editor who wanted to save space in setting up type so he made up the idea that the "and" replaced the comma. However, there is no basis for this claim in the grammar of English. It has merely been repeated for a long time as if it were true. It isn't.
What is the reason for putting the comma in? The answer requires a look at how the "function" words work, those words that have little meaning but show relationships between the "content" words in English.
The word "and" is a coordinator that joins words or phrases or sentences. What is on one side of the coordinator must be grammatically the same as what is on the other side: word and word; phrase and phrase; sentence, and sentence. (Notice the ",and" between two sentences. Here the comma shows that this is a sentence-level coordination. Also, the words "or" and "but" function in a similar way. )
When you put together words or phrases, you create a new grammatical unit. The items on both sides of the "and" are no longer grammatically separate. They are joined by the "and." If you leave out the final comma in the series, you are actually coordinating the final two units and creating a new entity. That is how the grammar works.
Writers of certain styles have adopted their own rules in defiance of this grammatical fact. Some businesses even devise rule books about language that all employees have to follow even if they know the rules to be incorrect. The argument for doing so is usually of the "we do it that way because we have done it that way" variety. On the other hand, virtually every college writing handbook now recommends the final comma before the coordinator though a few of them waffle a bit.
What of the claim that the "and" replaces the comma, making it unnecessary? In fact, the opposite is true: You can omit the "and" and not the comma. Doing so creates a series like this: red, white, blue. This variation on the normal series is called "asyndeton" (meaning "no coordinator"). Yet another variation on the normal series looks like this: red and white and blue. This variation is called "polysyndeton" (meaning "many coordinators"). Some writers who use this variation mistakenly include commas. In polysyndeton, the "ands" indeed replace the commasbut all of them. In some writing, the ampersand (&) is used in place of the "and." The ampersand is a clear signal that marks the end of a series and can be seen as equivalent to comma+and, but it is seldom seen in formal writing.
Certainly, there are some series in which the items are so different that nobody would get the wrong idea about the units, but you will never be wrong if you put the comma in.You can leave the door to your home open for a long time, and you may not get robbed, but you will never do the wrong thing by locking your door. So, if you want to be a clear writer, put that comma in. If someone argues with you about it, refer them to this page. Ask for proof from the grammar of English that the comma is not necessary. Ask for some evidence to refute the above information.
Recommendations to this department are welcome. If you have a feature of punctuation you would like to see discussed, send a message to firstname.lastname@example.org.