Comma Guide

I find it helpful to think of a comma as a pause. However, this is not a steadfast rule, and the nine guidelines below should not be considered rules either.

1. After introductory clauses

One of the most common comma mistakes is missing the comma after an introductory clause.

Incorrect: Before I start the research paper I should finish this level of the game.
Correct: In deference to the needs of shorter moviegoers, our theaters offer stadium-like seating and a strict no-hat policy.

2. Before a conjunction

If the subject of the sentence changes, you should include a comma and a conjunction (and, but, so, though, yet) to indicate the change in subject.

Incorrect: They were ostensibly there to discuss the case but she knew it was a date.
Correct: They were ostensibly there to discuss the case, but she knew it was a date.

However, if the subject of the sentence doesn’t change, then the comma isn’t necessary.
Correct: The two of them developed an instant rapport and soon became friends.

If you have a change in subject but are missing the comma and conjunction, then this error is the definition of a run-on:
Incorrect: The dog seemed impervious to training it never stopped barking.

This one possible correct version, using both the comma and the conjunction:
Correct: The dog seemed impervious to training, and it never stopped barking.

Note: if the conjunction is “because”, then the comma is not necessary:
Correct: The dog seemed impervious to training because it never stopped barking.

A comma splice is an error where the comma is present but the conjunction is missing.
Incorrect: The dog seemed impervious to training, it never stopped barking.

In order to fix this problem, you could add a conjunction, or if you really want the two parts of the sentence to be directly connected, you could use a semi-colon:
Correct: The dog seemed impervious to training; it never stopped barking.

3. Interjections

Set off an interjection by using the commas as the breaks before and after the interjection.
Correct: There were nine different challenges inside the maze, each more deadly than the previous one, but her resolve to finish was strong.
Correct: The artist’s earlier sculptures, however, were clearly derivative of those created by her former mentor.

In the previous sentence, the “however” was used as an interjection in the middle of the sentence, but “however” can also be used to combine two sentences in the following manner:

Correct: The artist claimed her sculptures were original; however, they were clearly derivative of those created by her former mentor.

4. Before “which” (and not before “that”)

If you want to pause, then use “which” and include a comma beforehand. If you don’t want to pause, and instead want to indicate that what follows is part of the description, then use “that” and don’t include a comma.
Correct: I like the new chair, which is made of leather.
Correct: I like the new chair that is made of leather.

5. Two or more adjectives

The comma goes between the adjectives.
Correct: She decided to appropriate time every day for the sole purpose of exploring some of the deepest, darkest corners of the Internet.

6. Quotes

The comma goes on the left.
Correct: She said, “the comma goes on the left,” and then she provided a lame example.

7. Place & Date

For places, a comma goes after the town and after the state. For dates, the comma goes after the day and after the year.
Correct: The Chicken Little Society of Funkytown, New York, was formed to assuage the fears of those who believe that the world will end on December 21, 2012.

8. The Ending Shift

An abrupt change at the end of the sentence, as in the first example below, should be set off with a comma. Also, an additional clause at the end of a sentence should be set off with a comma, especially for -ing verbs, as in the second example.
Correct: We were hoping that our trip would have an auspicious beginning, not an ominous one.
Correct: By the time the office manager had figured out what had happened, the payroll accountant had absconded with millions of dollars, leaving behind only an empty apartment and an old car.

9. List of three or more

In this article by Grammar Girl, she argues that the serial comma should be used, except in journalism, where the idea is to take up as little space as possible. I also support the use of the serial comma, not because it is the “right” way, but because the reader cannot tell from the “and” alone if it is the end of the sentence or not. The Wikipedia page on serial commas provides some specific examples.
Correct: I like dogs, cats, and birds.

References

In creating this lesson, I started with a list of uses for the comma that I had put together using various Internet sources and the book, Elements of Style by Strunk & White. This time around, I made some major changes and then visited the following websites, mainly to see how other comma guides have categorized the various uses of commas: