Series Comma

To preclude ambiguity, every element in a series should be preceded by a comma: The boy liked pizza topped with lettuce, mustard, relish, and onions. If the comma before the final element of the series is omitted, ambiguities often result, as in the following sentence: His lover's will left him all of her investments, stocks and bonds. Without the series comma between the second and third elements, we cannot know if she gave him all of her investments plus her stocks and bonds, or whether she left him only that portion of her investments consisting of stocks and bonds.