In part 1 of “Learn to Use Commas Correctly,” I discussed the importance of correct comma use. I also wrote about using commas between equal modifiers. Lastly, I also gave you two useful tests to employ when deciding if you need commas.

Part 2 discusses using commas with nonessential words or phrases, between independent clauses, and with a series of items.

Use a comma to set nonessential words or phrases apart from the rest of the sentence

     Mark Jones, who graduated from the University of South Florida, sold 10,000 copies of “The College Graduate’s Survival Guide.”

The phrase “who graduated from the University of South Florida” isn’t essential to the sentence. Without it the sentence still makes grammatical sense. All the phrase does is provide more information. It’s a nonessential phrase, so we need commas around it.

But if we alter the sentence slightly all the phrases are essential, and no commas are needed.

     Mark Jones graduated from the University of South Florida and sold 10,000 copies of “The College Graduate’s Survival Guide.”

Failing to place commas around a nonessential phrase may change a sentence’s meaning.

     Book editors, who don’t make much money, shouldn’t have to pay income tax.

That sentence means there aren’t any book editors who make much money. Therefore, they shouldn’t have to pay income tax.

But removing the commas gives a different meaning.

     Book editors who don’t make much money shouldn’t have to pay income tax.

Now the sentence means only those editors who don’t make much money shouldn’t have to pay income tax. Other book editors, those who do make good money, should have to pay.

Use a comma between independent clauses

Independent clauses can stand as sentences by themselves. When used together in one sentence they need a comma, along with a conjunction such as and, for, but, or, so, or yet.

     It's raining hard today, and tomorrow's forecast calls for more of the same.

Both “It's raining hard today” and “Tomorrow's forecast calls for more of the same” are complete sentences; therefore we need the conjunction and the comma. (As an aside, we could also replace the conjunction and the comma with either a period or a semicolon.)

Use a comma with a series of three or more items

     The weather forecast calls for high winds, rain, and possible hail.

You probably noticed I have a comma after rain in the above sentence. This comma is called the Oxford comma, and it’s used after the next-to-last item in a series of three or more items. It goes before the and or or. The Oxford Comma gets its name from the Oxford University Press style guidelines. However, not all stylebooks require the Oxford comma. The Associated Press Stylebook recommends it not be used. But the AP says writers should use the Oxford comma when one or more items in the series contains the word and.

     Police said Jones robbed banks, credit unions, and savings and loans.

That sentence is clearer than this one:

     Police said Jones robbed banks, credit unions and savings and loans.

The Oxford comma is controversial. I’m convinced that the world will someday end when two countries who possess yet-undreamed of weapons go to war over the use or non-use of the Oxford comma. For consistency, I use the Oxford comma in all series of three or more items.