Standard Accepted Guidelines for Comma Usage
Based on the Chicago Manual of Style, APA Style Manual, Bedford Handbook, Harbrace College Handbook, and Words into Type.

This is not a comprehensive list—just the most common usages you might run across. As a writer, you can choose whether or not to apply these rules to your writing. Just make sure whichever you choose to do, do it consistently. Don’t use a serial comma sometimes and not others. Either always use it or never use it. That way, when an editor sees your work, they will know you have made a deliberate choice instead of thinking you don’t know grammar well.

1. The Serial Comma. In a list of three or more items, a comma should precede the and:
          Please go to the store and get apples, bananas, and pears for the fruit salad.
          We came, we saw, and we conquered.
She got up, got dressed, brushed her teeth, put on makeup, dropped the overdue books off at the library, went to the grocery store, got gas, and arrived home before her kids got out of the bed.

Exception 1: Do not use commas when all items in a series are joined by a conjunction:
          We are going to Bermuda or Jamaica or Barbados. 

Exception 2: No comma is used with an ampersand (&):
          He stepped into the offices of Folse, Bordelon & Guidry.

2. Independent Clauses. When two independent clauses (two complete thoughts that could stand alone as sentences) are joined by a conjunction (and, but, or, nor, for, so, yet, if, because*), a comma precedes the conjunction.
          The reporter turned in her story, but she missed the deadline.
          We didn’t get to go to the park, because* it was raining by the time Dad got home.
          Do you want to go swimming, or do you want to go horseback riding?

Exception 1: If the clauses are very short and very closely related, no comma is needed:
          She knelt down and she prayed.

Exception 2: If one or both of the clauses contain internal commas, use a semicolon before the conjunction:
          If you want to continue working here, Jim, we would like to keep you; but we can no longer put up with your greasy hair, ratty jeans, and Moses sandals that show off your nasty toe-jam.

*Because is a recent addition to this list and currently has the status of a style choice rather than a rule. The rule used to state that no comma came before because in a sentence, whether or not what followed it was an independent clause. However, by definition, because is a conjunction and most copy editors are beginning to treat it as such. Be careful, though, that you do not confuse it with because of, which is a preposition.

3. Introductory Phrases. Use a comma after an introductory phrase at the beginning of a sentence. These are typically adverbial (beginning with adverbs such as before, after, never, always, not, very, or –ly words) or participial (beginning with the participial form of a verb) phrases:
          At the stroke of midnight, the coach turned back into a pumpkin.
Hoping to stop the horses, he jumped on the near one’s back and pulled the reins as hard as he could.

Exception 1: A single word or very short (2–3 words) phrase does not require a comma unless a pause is intended (when read aloud) or to avoid misreading.
         Before eating, the family always says grace. 
(not Before eating the family . . .)
          Before eating we always say grace.

Exception 2: A comma is not used after an adverbial or participial phrase that immediately precedes the verb it modifies (in other words, the phrase becomes the subject of the sentence):
          Out of the cave came the most horrific shriek.
Growing in the median are some pretty wildflowers.<

4. Oh, Ah, Yes, No, Well, and Direct Address.  A comma follows the exclamatory oh or ah at the beginning of a sentence (before and after if it comes in the middle). A comma follows yes, no, well, or other such words at the beginning of a sentence. A comma precedes and/or follows a name/title used in direct address.
           It is, oh, such a wonderful thing!
Well, I thought so.
What is it, Lassie? Timmy fell down the well?
I wonder, sir, if you would please refrain from stepping on my foot again.

Exception 1: No comma needed if it is a short phrase:
          Ah yes! Oh no! Oh well.

Exception 2: No comma is needed with the poetic O:
          O Lord, how wonderful are thy ways.

5. Restrictive and Nonrestrictive Phrases. This element includes Parenthetical Elements, Interjections, Dependent Clauses, Relative Clauses, Appositives, Not…But. If a phrase is Restrictive (i.e., necessary to the meaning of the sentence) it should not be enclosed in commas. If a phrase/clause is Nonrestrictive (i.e., supplemental information, can be left out without changing the meaning of the sentence), it is enclosed in commas.
          Brandilyn Collins, the best selling author, will be here for a book signing tomorrow. (nonrestrictive)
          Richard Armitage the British actor is young and good looking. Richard Armitage the former U.S. Deputy Secretary of State is older and not so good looking. (restrictive)
          Starbucks, which has really expensive coffee, is a national chain. (nonrestrictive)
          The book that I had to read for class is due back to the library today.(restrictive)
           (Which is typically nonrestrictive, That is usually restrictive)
          My sister, Michelle, is two years older than me. (nonrestrictive—I only have one sister)
          My mother’s sister Rinn lives in Florida. (restrictive—her sister Becky lives in North Carolina)
          It is, indeed, the most wonderful time of the year. (interjection)
          This, I think, is where we turn. (interjection)

6. Coordinate and Cumulative Adjectives. Remember these from the quiz? Coordinate adjectives are those whose order can be changed (or that can be joined by and) without changing the meaning of the phrase and they need a comma. If changing the order of the adjectives changes the meaning of the phrase, they are cumulative (or compounding) adjectives and do not need a comma.
          He was a mad, bad, dangerous-to-know man.
He was mad, bad, and dangerous to know.
We marveled at the enormous blue diamond necklace.

Repeated Adjectives also get a comma:
          You’re a bad, bad boy.

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Glenn, C., & Gray, L. (2010). The hodges harbrace handbook. (17th ed.). Boston: Wadsworth Pub Co.

Hacker, D., & Sommers, N. (2010). The bedford handbook. (8 ed.). Boston, New York: Bedford/St. Martins.

Press, U. O. C. (2006). The chicago manual of style. (15 ed.). Chicago, London: University Of Chicago Press.

Skillin, M. E., & Gay, R. M. (1974). Words into type. (3 ed.). Upper Saddle River, NJ: Prentice Hall