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1. In a series. This is the serial comma (a.k.a. the Oxford comma) and precedes the conjunction in a list of three or more items.
Example: Jennifer had vegetable soup, a turkey sandwich, and onion rings for lunch.
2. Dates. When the format of month, day, and year is used, a comma comes after the day (April 20, 2016).
3. Addresses. A comma is used between the city and the state (Boston, Massachusetts)
4. Degrees or titles used with names (Exception: commas are not used around Jr. or Sr.)
Example: David Smith, M.D., was the attending physician.
5. Direct address (a.k.a. vocatives)
Example 1: Put the book down, Nancy.
Example 2: Jim, what are you doing?
Example 3: Sit down, Brandon, and have a drink.
6. After introductory words and phrases or dependent clauses (Exception 1: When the introductory phrase is three words or less, the comma may be omitted if the omission does not create confusion. Exception 2: When the adverb or adverbial phrase modifies the whole sentence rather than a specific element of the sentence, a comma is needed.)
Example 1: After it stops raining, I’ll take the dog for a walk.
Example 2. Sadly, the woman didn’t survive the accident.
Example 3: Yes, I will help you.
Example 4: Reaching for her phone, she realized she left it in the car.
7. To set off nonessential (or nonrestrictive) elements
Example: The boys, not the girls, ate all the cookies.
8. Between independent clauses (complete sentences) joined by a coordinating conjunction (for, and, nor, but, or, yet, so)
Example: Katerina wanted to go to the party, but she had nothing to wear.
9. Appositives: if the words that follow a noun and describe it are nonrestrictive, set them off with commas
Example 1: Kimberly Johnson, lover of animals, volunteers at the animal shelter.
Example 2A: Cassidy’s brother, Thomas, hasn’t been home in years. (Cassidy has only one brother.)
Example 2B: Cassidy’s brother Thomas hasn’t been home in years. (Cassidy has more than one brother, but the one named Thomas hasn’t been home in years.)
Example 3A: Ben’s sister, wearing green shorts, is running on the track. (Nonrestrictive. Ben’s sister just happens to be wearing green shorts.)
Example 3B: The woman wearing green shorts ran into me. (Restrictive. The woman is defined, or restricted, by her green shorts.)
10. Between coordinate adjectives (adjectives that equally modify the same word).
Test 1: If “and” can be placed between two adjectives and the order can be changed without modifying the meaning, separate them with a comma.
Test 2: When each adjective alone can modify the noun and still make sense, separate the adjectives with a comma.
11. Nonrestrictive phrases and/or clauses, also known as parenthetical elements, should be set off by commas.
Example: The baker, who was frosting a cake at the time, didn’t notice the young boy taking the cupcakes.
12. Asides and interrupters are set off by commas
Example 1: Johnny was, as you can imagine, terrified when he saw the growling wolf.
Example 2: Brittany will, of course, handle the arrangements.
13. To set off phrases that modify something other than the preceding word
Example: Jenny threw herself on the couch, sobbing uncontrollably.
12] Between coordinate adjectives: adjectives that equally modify the same word.
Test 1:: when the word “and” could be placed between two adjectives and the adjective order can be swapped without modifying the meaning,
separate them with a comma. (Note: the test is not whether or not it sounds awkward, but does it change the meaning.)
Test 2: if each adjective alone could modify the noun and still make sense, separate the adjectives with a comma.