There’s been outrage in the US that a teacher used the image below to illustrate the importance of using the so-called Oxford comma.
What is an Oxford comma? This punctuation mark derives its name from the way the comma is used at Oxford University Press, who describe it as “a hallmark of OUP house style”. It is used before the coordination conjunctions and and or in a list of three or more items, as in the first sentence in the image. Those in favour of using the Oxford comma say that not using the comma, as in the second sentence, can cause ambiguity, and hence unintended meanings. Note that the second sentence can mean either that four people were invited, or just two, namely JFK and Stalin.
Most of the time there is no ambiguity if the Oxford comma is not used, so it boils down to a matter of taste whether or not you use it.
However, sometimes the Oxford comma does matter. In the literacy professional skills test, taken by prospective teachers in the UK, one of the tasks involves inserting punctuation in a text, and some candidates may wonder whether or not the examiners require the use of an Oxford comma. The instructions read:
You will be asked to insert punctuation into a piece of writing. You are not required to remove or rewrite any sections of the passage. The passage may contain instances where punctuation is acceptable although not essential. Any non-essential punctuation that is added to the passage will not be scored.
Since opinions differ as to whether the Oxford comma is essential or not, this instruction could be clearer to avoid anxiety.