The serial comma is the comma used immediately before the and or or preceding the final item in a list of three or more items. I wrote about the serial comma in this 2010 post, but I revisit it now because something caught my eye in Garner’s Modern American Usage.
Here’s what it says on page 676:
Whether to include the serial comma has sparked many arguments. But it’s easily answered in favor of inclusion because omitting the final comma may cause ambiguities, whereas including it never will ….
I think that’s incorrect, in that in addition to allowing you to avoid ambiguity, the serial comma can in certain circumstances cause ambiguity. Here’s what MSCD 12.61 says:
But the serial comma can also create ambiguity. Consider the following adjusted version of the dedication [discussed in the preceding paragraph]: To my mother, Ayn Rand [,] and God. With the serial comma, the reader could understand the dedication as meaning either that the book is dedicated three ways or that the book is dedicated to the writer’s mother, who happens to be Ayn Rand, and to God. Omitting the serial comma makes the latter meaning less likely.
Of course, whenever presence or absence of a comma can change meaning, it would be prudent to restructure the sentence to avoid that nuance.
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