One of the most widely known grammar rules is ‘don’t use a comma before and‘. Unfortunately, while this applies most of the time, there are a number of cases where we must use a comma before and. The rule really should be ‘don’t use a comma before and, unless it is needed to make your meaning clear’.
Commas are used to separate groups of words within a sentence to show which words belong together; this helps us to understand what we are reading. Since and is used to link things together, it makes sense that there shouldn’t be a comma before and, as the effect would be to separate what comes after the and from what comes before it, which is the opposite of what using and is meant to do. For example: For breakfast I had orange juice, muesli and a cup of tea. The and shows that a cup of tea is the last item in the number of things I had for breakfast. If the sentence had been For breakfast I had orange juice, muesli, and a cup of tea, we could read that as if the sentence is incomplete: that a cup of tea starts a new thought and the sentence should perhaps have been For breakfast I had orange juice, muesli, and a cup of tea would have been nice if I had had time.
The last example sentence shows when we do need to use a comma before and: when we need it to mark clearly the end of one thought and the start of another. Consider this sentence: The decision was made by the directors and senior management and the employees were unhappy. Now try to answer these questions: who made the decision? who was unhappy? We cannot tell, because the sentence is missing the vital commas that would enable us to understand it properly. The sentence should either be The decision was made by the directors, and senior management and the employees were unhappy or The decision was made by the directors and senior management, and the employees were unhappy. As readers, we cannot tell which meaning is meant; only the person who wrote the sentence can put the commas in to make clear what they meant.
A comma can also be needed before and in a list (as well as between two clauses, or separate thoughts). For example: The hotel menu offered many choices for breakfast: cereal, fruit and yoghurt, eggs and bacon or smashed avocado, and juice. The last comma means that there is choice between eggs and bacon and eggs and smashed avocado; if we leave the comma out, it means the eggs only come with bacon, and guests have a final choice between smashed avocado and juice.
This last example of using a comma before and – where the and is used in a list – is known as a serial comma. It is sometimes also called the Oxford comma. Just to make things more confusing, Americans always use a comma before and at the end of a list; they are serial users of the serial comma. Australians (and the British) do not use serial commas, unless we need them to make our meaning clear.
The serial comma: not a row of commas but a comma used before and in a list
The fact that two different rules exist in different parts of the world about using commas before and probably tells us that you shouldn’t worry too much about it!
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