Rules for Comma Usage
Use a comma to separate the elements in a series (three or more things), including the last two. "He hit the ball, dropped the bat, and ran to first base." You may have learned that the comma before the "and" is unnecessary, which is fine if you're in control of things. However, there are situations in which, if you don't use this comma (especially when the list is complex or lengthy), these last two items in the list will try to glom together (like macaroni and cheese). Using a comma between all the items in a series, including the last two, avoids this problem. This last comma—the one between the word "and" and the preceding word—is often called the serial comma or the Oxford comma. Use a comma + a little conjunction (and, but, for, nor, yet, or, so) to connect two independent clauses, as in "He hit the ball well, but he ran toward third base." One of the most frequent errors in comma usage is the placement of a comma after a coordinating conjunction. Use a comma to avoid confusion.
• English Grammar & Style