Commas, List, Including Oxford

A list consists of three or more grouped items: “bread and cheese” is a grocery list but not a grammatical list; “bread, cheese, and pickles” is both.

When constructing a list, place a comma between each item:

Variables analyzed in the regression included population size, GDP, third-party assistance, and number of previous endogenous conflicts.

That last comma—the one before the “and” (or “or”) that precedes the last item in a list—is known as the “Oxford” or “serial” comma. Ironically, perhaps, the Oxford comma is less frequently used in British English than American, and it is technically optional, provided you’re consistent within a given document.

Better safe than sorry

That said, leaving out the Oxford comma can potentially generate confusion (and chuckles) in your reader. Take, for example, this sentence:

This thesis is dedicated to my parents, Professor Chakrabarty and Professor Moustakas.

Did your parents advise your thesis?

The slip in meaning arises here because the comma is overworked: we could read this sentence either as a list with no serial comma or as apposition—a construction in which something is renamed. Compare this version, which clears things up:

This thesis is dedicated to my parents, Professor Chakrabarty, and Professor Moustakas.

Now it’s certain that we’re looking at a list.

Always including the Oxford comma is a useful way to minimize ambiguous list constructions and reserve that precious brainpower for other writing-related conundrums.

But wait

The Oxford comma is not a silver bullet for preventing ambiguity, however:

This thesis is dedicated to my kids, Sylvia, Hart, and Ezra, as well as my parents.

Are Sylvia, Hart, and Ezra the children here, or are they some other people this author wanted to thank? Again, are we looking at an appositive or a list?

In cases like this one, it can be useful to supplement commas with dashes or semicolons or other structural markers, which can clarify the boundaries between list items:

• This thesis is dedicated to my kids—Sylvia, Hart, and Ezra—as well as my parents.
• This thesis is dedicated to my kids; to Sylvia, Hart, and Ezra; and to my parents.

Finally, consider simply rearranging the list items:

I went on a walk with my dog, Lewis, and his sister.

Unless you’re packing multiple leashes and plenty of doggy bags, we’ll probably want this version:

I went on a walk with Lewis, his sister, and my dog.

Lists vs. Nested Pairs

One particularly common comma error is treating a nested pair as a list:

Chapter I explains the research problem, methods, and discusses relevant literature.

This sentence is not a list; rather, it contains a pair of verbs (explains and discusses), one of which distributes to a pair of direct objects (research problem and methods). These pairs should each be treated in the usual way—connected with “and,” with no commas:

Chapter I explains the research problem and methods and discusses relevant literature.

In constructions like this one, writers might be tempted to retain the comma before the second "and": "explains the research problem and methods, and discusses relevant literature." But, for the same reason, this comma, too, is unnecessary.

Conclusion

As with any other construction, achieving clarity in your lists requires being attuned to possible misreadings and quashing them with punctuation, restructuring, and the rest of your syntactic toolkit.

List Commas Links

• GWC writing mechanics module: Commas
• Video (1:51): "Punctuating a List," Khan Academy
• Video (3:33): "Bonus: the Oxford comma," Khan Academy
• Handout (printable): "Commas," University of North Carolina Chapel Hill