Notes

1. 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. In newspaper writing, incidentally, you will seldom find a serial comma, but that is not necessarily a sign that it should be omitted in academic prose.


2.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."

Contending that the coordinating conjunction is adequate separation, some writers will leave out the comma in a sentence with short, balanced independent clauses (such as we see in the example just given). If there is ever any doubt, however, use the comma, as it is always correct in this situation.

One of the most frequent errors in comma usage is the placement of a comma after a coordinating conjunction. We cannot say that the comma will always come before the conjunction and never after, but it would be a rare event, indeed, that we need to follow a coordinating conjunction with a comma. When speaking, we do sometimes pause after the little conjunction, but there is seldom a good reason to put a comma there.
whistler.

3

Use a comma to set off introductory elements, as in "Running toward third base, he suddenly realized how stupid he looked."

It is permissible to omit the comma after a brief introductory element if the omission does not result in confusion or hesitancy in reading. If there is ever any doubt, use the comma, as it is always correct. If you would like some additional guidelines on using a comma after introductory elements, click HERE.
4

Use a comma to set off parenthetical elements, as in "The Founders Bridge, which spans the Connecticut River, is falling down." By "parenthetical element," we mean a part of a sentence that can be removed without changing the essential meaning of that sentence. The parenthetical element is sometimes called "added information." This is the most difficult rule in punctuation because it is sometimes unclear what is "added" or "parenthetical" and what is essential to the meaning of a sentence.

Appositives are almost always treated as parenthetical elements.

Calhoun's ambition, to become a goalie in professional soccer, is within his reach.
Eleanor, his wife of thirty years, suddenly decided to open her own business.