QUESTION
Should I put a comma before last item in list?
I would like crackers, cheese and some soda.
I would like crackers, cheese, and some soda.
ANSWER
Using a comma before the last item in a list is known as the Oxford Comma, sometimes the Serial Comma. It is used before conjunctions in a list like this with three or more items. Its use is purely written style and optional. It is more common in America outside journalism, and much less common in other English speaking areas of the world. There are arguments for and against which usually come down to comprehension. Wikipedia quotes these ambiguities:
To my parents, Ayn Rand and God.
To my mother, Ayn Rand, and God.
Also on that wiki page you can find lots of links to certain style guides. Comma use is something of a grey area though, and everyone has his own style. Pick what reduces ambiguity.
Language log has an interesting article on how reading comprehension can be improved with comma use, including this type.
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