Commas may be the most commonly used punctuation mark. They also may be the most misused. 

The problem of correct comma use is compounded by conflicting advice. For example, in the next paragraph I use a comma before but. However, Microsoft Word tells me to take it out. 

Learning to use commas can be challenging, but using them correctly is critical to clear writing.

The reality is that leaving the comma in or taking it out doesn’t make much difference. However, here’s an example of a comma being critical to clarity:

     Let's eat, Grandma!

That sentence brings up pleasant connotations about the family gathered around the Thanksgiving table. Your senses are on overdrive with the sight and smell of the almost-ready turkey, stuffing, and mashed potatoes, not to mention the pumpkin pie. You're with your loving family and everything is warm, fuzzy, and happy. So, it's “Let's eat, Grandma!”

Now try the same sentence without the comma:

     Let's eat Grandma!

Now Grandma isn't cooking the turkey; Grandma is the turkey. And suffice it to say that Grandma has little interest in being consumed for dinner. Yet that's exactly what that sentence is saying, merely because I removed a comma.

As in the above example, some writers leave commas out with unfortunate results. On the other hand, some writers throw commas in at random, as in this sentence from a condominium association monthly newsletter: 

     The board, bids out work to existing contractors as well as potential suppliers.

There is absolutely no reason for the comma. 

Inserting or leaving out commas may even give the same sentence opposite meanings.

     Joe said Judy is a wonderful person.

That’s clear enough—in Joe’s opinion, Judy is a wonderful person. But let’s write the same sentence with commas.

     Joe, said Judy, is a wonderful person.

Now it’s Joe who, in Judy’s opinion, is a wonderful person.

Now that we’ve established that correct comma use is critical, let’s see some rules for using them. Let’s tackle one of the most difficult concepts first—commas with equal modifiers.

Use commas between equal modifiers 

Some writers and editors have difficulty knowing when to use commas in a string of two or more modifiers. For example, do we need the comma between competent and effective in this sentence? 

     People who work hard at learning to edit become competent, effective editors 

The answer is yes, but how do we know? The uncertainty arises over the issue of whether the modifiers competent and effective are equal. They are, but how do we know? 

Here are two useful tests to employ when making this decision:

1. Can we switch their order and still make sense?

     People who work hard at learning to edit become effective, competent editors.

That makes sense; it works. 

The other test is:

2. Can we link the modifiers with the conjunction and without changing the sentence’s meaning?

     People who work hard at learning to edit become competent and effective editors.

That also works, so in this example, either test works. The sentence still makes grammatical sense if we switch the order. It also works if we use and between them. Therefore, we know the modifiers are equal and we need a comma between them.

Note that a sentence only has to pass one of the two tests; if it does, we need a comma between the modifiers.

What about this sentence? Are the modifiers three and overpaid equal?

     Making the book marketable required the work of three overpaid editors.

Let’s apply the two tests. Can we switch the order?

     Making the book marketable required the work of overpaid three editors.

That doesn’t work. What about using and in place of a comma?

     Making the book marketable required the work of three and overpaid editors.

That doesn’t work either. The sentence fails both tests, therefore no comma is needed.

Because of the importance of this point, let’s look at one more sentence. (This one’s a bit tougher, as you’ll see in a moment.)

     Two harried overworked book editors left the meeting early.

Let’s apply the “switch the order” and “add and” tests. 

     Overworked two harried book editors left the meeting early.

That doesn’t work; the test fails. So, let’s apply the second test, adding and. 

     Two and harried and overworked book editors left the meeting early.

That doesn’t work, either. Because both tests fail, it appears we don’t need commas. Let’s try it:

     Two harried overworked book editors left the meeting early.

But that doesn’t look right, does it? It's not right, and here's why: In this sentence, two of the modifiers are equal; one is not. Take another look at harried and overworked. Can we switch their order? 

     Two overworked harried book editors . . . 

Well, that works, sort of. But let’s try the other test: can we put and between them?

     Two harried and overworked book editors . . . 

That works! Therefore, we don’t need a comma between two and harried, but we do need a comma between harried and overworked. 

     Two harried, overworked book editors left the meeting early.

You’ll find the two tests quite useful in deciding when to use commas. 

Part 2 of “Learn to Use Commas Correctly” discusses using commas with nonessential words or phrases, between independent clauses, and with a series of items.