Rule 4b. Use commas to set off expressions that interrupt the sentence flow (nevertheless, after all, by the way, on
the other hand, however, etc.) or present a contrast. Also use commas with dates, locations, and titles, as in
these examples:
Example: I am, by the way, very nervous about this.
Example: I can go, can’t I?
Example: Will you, Aisha, do that assignment for me?
Example: It was in the newspaper’s June 5, 2003 edition.
Example: He grew up in Akron, Ohio.
Example: Al Mooney, M.D. is here.
Rule 5. When starting a sentence with a dependent clause, use a comma after it.
Example: If you are not sure about this, let me know now.
But often a comma is unnecessary when the sentence starts with an independent clause followed by a
dependent clause.
Example: Let me know now if you are not sure about this.
Rule 6. Use commas to set off nonessential words, clauses, and phrases. If something or someone is sufficiently
identified, the description that follows is considered nonessential (it can be removed from the sentence without
fundamentally altering the meaning) and should be surrounded by commas. If the description is essential (it
cannot be removed), then do not use commas.
Example: My brother Bill is here.
Now, see how adding commas changes the sentence's meaning:
Example: My brother, Bill, is here.
Careful writers and readers understand that the first sentence means I have more than one brother. The
commas in the second sentence mean that Bill is my only brother. Why? In the first sentence, Bill is essential
information: it identifies which of my two (or more) brothers I'm speaking of. This is why no commas enclose Bill.
In the second sentence, Bill is nonessential information—whom else but Bill could I mean?—hence the
commas. In the following example, who has a limp is essential information and so no commas are used:
Example: The boy who has a limp was in an auto accident.
We do not know which boy is meant without further description; therefore, no commas are used.
Comma misuse is nothing to take lightly. It can lead to a train wreck like this:
Example: Mark Twain's book, Tom Sawyer, is a delight.
Because of the commas, this sentence states that Twain wrote only one book. In fact, he wrote more than two
dozen of them.
Following are two instances of the need for an appositive comma with one or more nouns. Don’t forget the
second comma:
Incorrect: My best friend, Joe arrived.
Correct: My best friend, Joe, arrived.
Incorrect: The three items, a book, a pen, and paper were on the table.
Correct: The three items, a book, a pen, and paper, were on the table.
Rule 7. Use commas to introduce or interrupt direct quotations.
Example: He said, "I don't care."
Example: "Why," I asked, "don't you care?"
Rule 8. Use a comma before and after certain introductory words or terms, such as namely, that is, i.e., e.g., and for
instance, when they are followed by a series of items.
Example: You may be required to bring many items, e.g., sleeping bags, pans, and warm clothing.
Rule 9. Use a comma before the term etc.; if it is placed midsentence, commas should enclose it.