“For decades, the drug-resistant germ called MRSA was almost exclusively a concern of humans…. But in recent years, the germ has become a growing problem for veterinarians, with an increasing number of infections turning up in birds, cats, dogs, horses, pigs, rabbits and rodents….”

“For protection, Dr. Oehler recommends hand washing or using hand gels before and after playing with a pet, not letting a pet lick people around the face, and not washing pet food or water bowls in the same sink that food is prepared in.”

– Brenda Goodman, “Tie to Pets Has Germ Jumping to and Fro,”
New York Times online, September 21, 2009

Do you see how Goodman uses commas in two different ways here? She leaves out a final comma in “pigs, rabbits and rodents.” Yet she does add a final comma before “and” in the last sentence.

Why? Because the Associated Press tells journalists to use commas in different ways for different kinds of lists. The last sentence isn’t a list of simple items like birds and rodents. The final list item includes a conjunction of its own: “not washing pet food or water bowls in the same sink that food is prepared in.” The list of three complex items would be confusing without a final comma before the list’s main conjunction: “and.”

We recommend that most writers always add a comma before the conjunction in all lists of three or more items. (Not “The flag is red, white and blue,” but “The flag is red, white, and blue.”) Then you’ll never have to ask yourself if you need another comma to make your meaning clear.

For more tips about commas, see Write It Well’s book Professional Writing Skills: A Self-Paced Training Program.