Use the Serial Comma

1. The toy was red [comma] round [comma] and heavy.
2. I purchased pickles at the store [comma] gas at the convenience store [comma] and flowers at the florist for my beautiful wife.

Series: A series is a string of three or more matching items in a sentence. For example, the series in sample 1 contains three items: red, round, and heavy. The series in sample 2 also contains three items. Take a look at sample 1 and see how the commas fit the rule. The first item is red, and it is separated from round by a comma. The second item is round, and it is separated from red and heavy by commas. Every item in the series is separated from the other items by commas.

3. I respect my parents, Fred and Gloria.
4. I respect my parents, the president and the first lady.*
5. I respect my parents, the president, and the first lady.

These three samples help to understand why we use a comma before the final and, but, or or. Let’s look at them carefully and see what they might mean to the reader.

In sample 3, the reader will most likely interpret this sentence to mean “I respect my parents.” The names of my parents are Fred and Gloria. The reader might think I respect my parents, I respect another person named Fred, and I respect yet another person named Gloria, but this is unlikely. The most likely interpretation is that Fred and Gloriaare the names of my parents. The sentence structure and comma use lead the reader to that conclusion.

Now look at sample 4. This has the same sentence structure and comma use as sample 3. How will the reader interpret this sentence? Well, the reader could interpret this to mean my parents are the president and the first lady, which is not true. We need the final comma used in sample 5 to make clear that we’re talking about 3 groups of people: (1) my parents, (2) the president, and (3) the first lady.

The comma before and, but, or or that introduces the final item is called the serial comma. It is also called the Harvard comma and the Oxford comma.

The Associated Press (AP) style guide, which governs most journalism writing, tells writers not to use the serial comma. This may be an attempt to save a little column space for words by removing punctuation. Nearly every other style guide says to use it—and for a good reason.

As we see from samples 3–5, the serial comma, or its absence, can affect how the reader interprets the sentence. When we leave it out, we increase the possibility that the reader will misunderstand the sentence. When we leave it in, we clearly identify each item in the series.

Here’s another example of how that comma makes a difference.

6. Susan, Fritz, and Tom and Julie have one dog each.

Ok, how many dogs are we talking about? Possible answers are 2, 3, and 4. If we put the comma after Tom, we know the sentence is describing only 3 dogs.

But what if the parts are obvious, such as in sample 1? Do we still need the serial comma?

Remember, the purpose of the comma is to help the reader find the meaningful parts in a sentence. Although we could remove it sometimes without confusing the reader, other times it is necessary to make our meaning clear. That brings us to a general principal for commas (and all punctuation): 

Using the serial comma is never going to be wrong (unless you are required to use the AP style guide), so be consistent.

While we’re on the topic of series, I’ll make another point to help you separate items in series.

Sometimes, the items may be complex, meaning they have their own commas. If any of the items have commas, use semi-colons to separate them, as follows.

7. This is my brother, Bob; his friend; and my wife.

Even if only one of the items has its own commas, separate every item with a semicolon.

Some word processors have an option to require the serial comma when checking grammar and spelling. I recommend you turn it on.

SUMMARY
Rule A: Separate every item in a series with commas.
Rule B: Use the serial comma.
Rule C:Use semicolons to separate items that have their own commas.

This content is from the writing guide Zen Commas, an instructional reference guide that will help you solve the number one punctuation problem: commas. Now available in Kindle and PDF at http://HostileEditing.com