The serial comma (also known as the Oxford comma) is surprisingly controversial. In this post, I will explain why and show you how to use it.
What Is The Serial Comma?
The serial comma is placed before the conjunction in a series (list) of three or more items, as in the following example. (Throughout this post, the places for potential serial commas are in green.)
Evelyn, Tony, and Rajiv had to decide whether to go skiing, hiking, or kayaking.
What Is The Debate About The Serial Comma?
Some authorities have stated that serial commas are (usually) unnecessary and thus should (again, usually) be omitted from lists. They have argued that the conjunctions ("and" & "or" in the example above) already signal that the last item in the list is coming up, so the comma there is unnecessary. Certainly, the sentence above is perfectly understandable without the serial commas:
Evelyn, Tony and Rajiv had to decide whether to go skiing, hiking or kayaking.
Other authorities have contended that omitting the serial comma often causes sentences to be ambiguous, confusing, or just straight-up hard to read. Consider these examples that lack serial commas:
The uniforms featured blue panels, black and white pockets and gold piping.
The attendees included Andrea Jackson, the mayor and my mother.
The first example is difficult to read because there are two uses of "and" in the list—one to separate the last two list items and one within the second list item. This makes it harder to determine the location of the division between the items. Though the reader should be able to figure out the writer's intent, doing so may require extra effort.
The second example is ambiguous; it could be read as listing three people or as stating that one attendee (Andrea Jackson) is both the town's mayor and the writer's mother. Without the serial comma, the reader may not be able to determine the correct meaning from context.
With the serial comma, however, the first statement is easier to read, and the second statement is clearly a list of three individuals:
The uniforms featured blue panels, black and white pockets, and gold piping.
The attendees included Andrea Jackson, the mayor, and my mother.
Because most sentences work fine without the serial comma, authorities that prioritize brevity (mostly journalistic style guides such as AP) prefer that it be omitted in most cases. Even these authorities concede that the serial comma should be used when necessary to ensure that the sentence is clear and readable. I'll talk more about these exceptions below.
On the other hand, most academic and professional authorities—including the APA, MLA, AMA, and Chicago style guides—recommend always using the serial comma. These authorities prioritize clarity over conciseness, and they recognize that an "always" rule is easier to understand and implement than a "most of the time" rule.
What Should You Do About The Serial Comma?
If you are writing informally, then it doesn't matter which rule you apply as long as you are consistent. There is no consensus on which rule is better. You can omit the serial comma in all cases (except when necessary for clarity or readability), or you can use it in all cases. Don't arbitrarily vacillate between using it and omitting it, though; choose a rule and stick to it.
If you are writing formally and have a style guide, always defer to its preference. You should be able to find the relevant section by looking up "Commas" in the guide's index. As noted above, journalistic styles tend to omit the serial comma, but academic and professional styles tend to require it.
If your style guide does not express a preference on the matter, or if you are not using a style guide, then I would recommend always using the serial comma. I have three reasons for this:
I value clarity above conciseness, and always using the serial comma helps ensure clarity.
This rule requires less thought ("cognitive load") to implement because you don't have to think about whether a list needs the serial comma—you just always use it.
A comma doesn't add much length to a sentence anyway!
If you aren't sure, you can also ask someone who will be evaluating your writing (e.g., your instructor, publisher, or editor). They should be able to provide some guidance.
When Do I Always Need To Use The Serial Comma?
Let's say that you've opted to use the serial comma only when necessary; perhaps you are using AP Style. When do you need to use the serial comma? Here are some situations to look out for:
When the sentence grammatically requires a comma, such as when each item in the list is a complete thought ("independent clause"). In this example, the comma is needed not because it is a serial comma but because commas always separate complete thoughts:
Alvin brought plates, Bonnie brought cups, and Camila brought forks.
When one or more of the individual items itself contains a conjunction. In this example, leaving out the serial comma makes the sentence harder to read because the last item is itself a compound ("shocks and struts"):
The car-parts store has a sale on brakes, tires, and shocks and struts.
When any of the individual items in the list could be confused with any other. In this example, leaving out the serial comma opens the possibility that the last two items could be read as renaming the first item:
I went to my favorite restaurant on July 4, last Friday, and my birthday.
Note: If you want to actually state that July 4 was last Friday and/or was your birthday, try rephrasing. For instance, you can use an intervening phrase to clarify:
I went to my favorite restaurant last Friday, July 4, which was my birthday.
If you have any doubts about whether the serial comma is necessary, just use it to be safe.
Do you have any questions about the serial comma, or anything else related to punctuation? Let me know in the comments or by email at firstname.lastname@example.org. Thanks for reading!