When there are two items in a list, don't use a comma to separate the list items (unless it helps your reader). If there are more than two list items, then those following US convention should use a comma before the conjunction (usually and or or).
Commas in ListsWhen there are two items in a list, there is no need to separate the list items with a comma.
When there are more than two items, the situation gets more complicated. Generally speaking, in the UK, writers tend not put a comma before the conjunction. In the US, however, writers tend to use a comma.
This delineation between UK and US is very general — there are plenty of organisations in both countries that do not adhere to this.
The Oxford CommaWhen a comma is used before the conjunction in a list, it is known as an Oxford Comma. Some people (mostly but not exclusively Brits) consider the Oxford Comma to be a waste of ink, while others (mostly but not exclusively Americans) strongly campaign for its inclusion. There really is no rule. You should follow whatever convention your company follows. If you don't have a company convention, then copy the convention used in a decent national newspaper. If you're free to make your own mind up, then pick a convention and be consistent. (That is the golden rule.)
There is another consideration. Sometimes, it is appropriate to break whatever convention you're following for the sake of clarity — even in a list with just two list items. Examples:
the and makes it easier for the reader to identify the last list item.)
Using Lots of Adjectives (Enumeration of Adjectives)Often in creative writing, there is a need to use several adjectives (describing words – see lesson Adjectives). The rules about using commas in a list of adjectives are far more relaxed. For example:
For TWO adjectives:
BEWARE CONFUSION WITH AN OXFORD COMMA
So, when a comma is used before a conjunction in a list, it could make the list item before the comma look like a parenthesis. This could create confusion. For example:
BE CONSISTENT – BUT BREAK CONVENTION FOR CLARITY
Follow one of the conventions, and stick to it throughout your document. However, if you write something ambiguous, try to reword your sentence. If that proves too cumbersome, have the confidence to switch conventions in the same document. Above all, remember this:
Clarity trumps both conventions.