Commas separate items in a list of three or more. There is debate on whether a comma should follow the penultimate item in such a list. This serial comma is popularly called the Oxford comma.
We have all seen hilarious examples of what can happen without the Oxford comma:
I looked up the Oxford comma in several language guides that I happen to have on hand (I am a pedant). Three of four (The Chicago Manual of Style, Garner’s Modern American Usage, and Strunk and White’s Elements of Style) unequivocally endorse its usage. CMOS states that it prevents ambiguity. Garner claims that, while omitting the comma can produce ambiguity, adding it “never will.” Strunk and White simply say that a comma is to be used after each item except the last.
Only the Associated Press Stylebook recommends against the Oxford comma’s usage in a simple series. (Garner points out that newspaper journalists typically leave out the comma to save space, which may explain the discrepancy.)
So, from my admittedly small sample of style and usage guides, most support the use of the Oxford comma, and those that justify their position point to ambiguities caused by the omission of the comma. They are concerned about clarity of meaning.
To that effect, the pictures above do an excellent job conveying the importance of a properly placed comma. It’s easy, though, to create specific sentences in order to get a desired effect. Allow me to put a new twist on an old picture:
The result would be the same if the sentence was: “We invited JFK, the stripper, and Stalin.” In both cases the removal of the comma would make things more clear to any reader who likes to imagine powerful men in sexy lingerie. However, ambiguities abound. In my pictured example, the sentence with the comma can mean that we invited three individuals—okay, that makes sense. The sentence without the comma can also be read to mean that the stripper was both JFK and Stalin. Of course, this interpretation rings absurd in most contexts. Consider, however, the following cast list for my new show:
The point is, we are constantly making judgments about what makes sense contextually (whether the context is provided by the rest of the text or by what we know about the world), and that informs our interpretation of a sentence. We can often make up a context which leads to a funny or absurd reading. It is important—and relevant to the discussion—that the people reading the sentence about strippers already know that neither JFK nor Stalin are actually strippers. If they didn’t know this, the image wouldn’t be so funny, and viewers would not be able to tell whether it was trying to advocate for or against the comma’s usage. In other words, the reason that image is effective as an argument for the Oxford comma is that people would never actually be confused by the absence of the comma in that context.
This is important because, as I’ve hopefully illustrated, it can be difficult to make writing completely free of ambiguities. We can revise or break up ambiguous sentences to clarify. In my example, the most obvious ambiguities can be removed by switching the order of the listed objects. We invited JFK, Stalin, and the stripper. With or without the comma, the meaning is clear, unless of course JFK and Stalin are actually the same person. Another option is to add a parenthetical to make clear if one of the list items is being described. We invited JFK (Stalin) and the stripper.
Context matters. Just about every grammar rule is actually a guideline that is suitable most of the time. We should not take for granted that simply adding an Oxford comma to a list will result in the best way to phrase it.