Commas are punctuation marks that perhaps cause the most trouble for many writers. The comma is used to separate things, ideas, or thoughts in a sentence. Some writers use too many, and some don't use enough.
- When connecting two independent clauses, use a comma and a conjunction: Henry paid the vendor, and he closed the shop.
- Introductory parts of a sentence should be set off with a comma: Shoving her tennis shoes into a bag, she stepped off the elevator just in time for the meeting.
- A parenthetical element, one that can be removed without changing the meaning of the sentence, should be set off with commas. If you remove the parenthetical element of the previous sentence, the sentence reads: A parenthetical element should be set off with commas.
- If you can put the word "and" between adjectives used to describe nouns, a comma can be used: Eleanor lives in an old and stately building. With the comma: Eleanor lives in an old, stately building.
- When quoting someone, use commas to set off the quoted material: According to the speaker, "Commas are the bane of many writers' existences."
- When showing contrasts in the sentence, use commas to separate them: Money, not evil, is the driving force for the bank robbers.
- To avoid confusion, use a comma. Without the comma: For many the quarterly report is enough. With the comma: For many, the quarterly report is enough.
- Never use only a single comma between a subject and its verb. Incorrect: Mary, ran to the supply room. Correct: Mary ran to the supply room. Using two commas to explain more details about a subject is okay, however: Mary, who usually moves slowly, ran into the supply room. Another way to think about this is that you always need two commas so the second cancels the first one out. With one, the sentence is incorrect because the subject and verb are separated; with two, the sentence is "reset" to being correct again with the subject and verb unseparated.
- Commas are often used between a city and state (Phoenix, Arizona), a date and the year (January 5, 2012), a name followed by a title (James Patterson, author), and in numbers with four digits or more (3,456), though the comma is sometimes omitted for four-digit numbers. The key here is to be consistent within a communication; if you use the comma in a four-digit number, make sure to use it for all cases of four-digit numbers throughout your document.
- Finally, use commas cautiously. Just because there is a natural pause when reading a sentence doesn't mean a comma is necessary. If you are not sure, read over the rules above and if you can apply one of these rules, go ahead and use the comma.
The Series Comma
The series comma, sometimes called the serial comma, or the Oxford or Harvard comma, is the comma that appears just before the "and" or "or" in a series. For example, in the sentence "John likes apples, oranges, and pears.", the comma after the word "oranges" is the series comma.
Some schools of thought believe that the series comma is unnecessary, while others will insist it be used. For example, news agencies generally follow AP style guidelines and do not use the series comma.
Many American businesses follow the Chicago, APA, or other style guidelines, where the series comma is recommended. Again, whichever rule you use, be consistent in your usage.