"I have spent most of the day putting in a comma and the rest of the day taking it out." - Oscar Wilde
If only business writers had the luxury of pondering weighty decisions such as when to use a comma.
Here are a few rules for comma use, followed in most cases by an explanation.
Serial comma: When you list a series of items, you separate them with commas. Whether or not you add that last comma before the last item is up to you and whether you're British or American. (The Brits favor the last comma. Maybe a serial comma got to Oscar Wilde.)
Although the last comma is often omitted by today's writers, it is always needed if there is a chance for confusion. My favorite sandwiches are ham and cheese, peanut butter and jelly and turkey. A comma after jelly would avoid anyone thinking I enjoy sandwiches that combine peanut butter, jelly, and turkey. It's also a good idea to use a serial comma when the elements of the sentence are long or complex: Jennifer wanted to visit the pyramids, participate in photographic safaris in Kenya and South Africa, and observe the wildlife in Tanzania. If a comma helps the reader understand, include it.
Put commas around any part of the sentence that is set off from the rest of the sentence. At the beginning of a sentence, you need only one comma.
"Eventually, the team decided on a mission statement."
"Annoyed by the interruption, I returned to my filing."
"The weather in Biloxi, Mississippi, is humid."
"His favorite part of the day, lunch, had to be postponed."
A comma should set off vocatives (direct address). "Martha, you are a noisy cat." "Find the toy, you little scamp."
Interjections such as yes, well and indeed should generally be separated from the rest of the sentence by a comma. "Well, I wish you'd told me yesterday." "His project is, indeed, over budget."
However, you should not put a comma after most beginning coordinating conjunctions. For, and, nor, but, or, yet and so are coordinating conjunctions. Many of us were taught that we should never begin a sentence with but or and. But there's no reason not to. (Yes, it's also OK to end that sentence with a preposition.) Should you choose to start your sentence with a but or an and, don't follow either with a comma.
When using the adverb too at the end of a sentence, you get to choose whether or not to precede it with a comma. That option holds true for also, however and therefore. If you wish to emphasize or indicate a clear break in thought, use a comma.
I have reserved for a later column the rules about appositives. If you can't wait, Daily Writing Tips and the Grammar Girl are online resources with good explanations. Rules for comma use are guidelines. Clarity trumps. So do corporate style guides (which have great value if they are kept up to date and easily accessible online).