The comma is the most abused punctuation mark, possibly because writers are sometimes so worried about following rules that they forget to pay attention to the way words sound when spoken. The function of a comma is to slow the reader down, briefly – to make the reader pause. The omission of a comma can allow phrases and clauses to crash into one another, thereby confusing the reader.
Commas can influence the meaning of your sentence. Consider:
Although I wanted to kill Max, I controlled myself.
Although I wanted to kill, Max, I controlled myself.
In the first sentence, Max is the person I wanted to kill; in the second sentence, I am talking to Max about my desire to kill something else. The comma controls the meaning.
Let’s look at another:
The food tastes terrible, however the cook fixes it.
The food tastes terrible; however, the cook fixes it.
In the first sentence, the food tastes terrible no matter how the cook fixes it. In the second sentence, the cook improves the taste of the food. Again, the comma controls the meaning.
Finally consider the title of a popular book about punctuation:
Eats Shoots and Leaves
Note how a comma changes the meaning: Eats, Shoots and Leaves
* Use a comma to separate two independent clauses connected by and, but, or, nor, for.
Bob was usually a quiet man, but he screamed upon entering the room.
The strange man lying under the table appeared to be dead, or just possibly he was only napping.
If the independent clauses are short, you may omit the comma.
The man was still and his foot was bleeding.
His hat was on but his pants were off.
* Use a comma to separate elements in a list or series; the comma is a substitute for “and.” In Associated Press style, omit the final comma.
* Use a comma to separate introductory phrases and clauses from the independent clause, particularly if the phrase or clause is long.
After catching his breath, Bob squatted next to the man and took his pulse.
When he felt nothing, Bob picked up the bassoon and blew.
Although he had never played a bassoon before, he somehow managed to make beautiful music.
If the introductory phrase is a gerund, participial or infinitive phrase, use a comma even if the phrase is short. Otherwise the reader may be confused:
When Bob began to eat, rats ran across the carpet.
NOT: When Bob began to eat rats ran across the carpet.
Before leaving, Bob heard the man sneeze.
NOT: Before leaving Bob heard the man sneeze.
* In a series of adjectives, use a comma if the adjectives could also be separated by “and.”
The nimble, fat and furry raccoon began to poke at the water balloon.
(Could write as: The nimble and fat and furry raccoon …)
If the “and” doesn’t fit, leave out the comma.
The man’s white cotton shirt was balled up in a corner.
(Wouldn’t write as: The man’s white and cotton shirt …)
Another test: If you can change the order of the adjectives, put in commas. For example:
The handsome, brilliant student
OR: The brilliant, handsome student
The frilly party dress
NOT: The party frilly dress
* Use commas to set off clauses, but don’t use commas for defining clauses. (Quick review: a defining, or restrictive, clause is one that can’t be left out of a sentence. Clauses that don’t define can be lifted from the sentence without changing the meaning. Also, a defining clause specified which part of a larger group we are talking about.)
Bananas that are green taste tart. (“that are green” defines which bananas we mean)
Bananas, which grow in the tropics, do not need refrigeration. (“which grow in the tropics” refers to all bananas. The clause can be lifted from the sentence without changing the meaning.)
Let’s look at a sentence you could punctuate either way, depending on the meaning:
The men who were tired and hungry began eating sardines. (“who were tired and hungry” is a defining clause, telling us which men we mean)
The men, who were tired and hungry, began eating sardines. (“who were tired and hungry” describes all of the men, and doesn’t differentiate these men from other men who weren’t tired and hungry)
* Words or phrases that interrupt the sentence should be set off by commas.
Now then, let’s get down to work.
On the other hand, error can lead to revelation.
What the candidate promised, in fact, is impossible to achieve.
I believe, yes, you should go.
* Use commas to set off an appositive. An appositive is a noun or pronoun that explains or introduced the noun preceding it.
Mrs. Dingdong, my favorite teacher, wears a wig.
Ralphie, the president of the student council, is on probation.
Thanks to: “Grammar Smart: A Guide to Perfect Usage” by the staff of the Princeton Review (Villard Books, 1994) and the Associated Press Stylebook.