Now let’s look at some basic rules regarding commas. Let’s start with Independent Clauses.
Always place a comma before a conjunction introducing an independent clause. In other words, place a comma between two independent clauses separated by a conjunction. Independent clauses have a subject and a verb, and they can stand alone. (The situation looked hopeless, but there was one remaining chance for success.) or (The situation looked hopeless, but I didn’t believe it.)
However, do not join independent clauses with a comma if they are lacking a conjunction. They need to be joined with a semi-colon, or they can be cut into two separate sentences. (The situation looked hopeless; there was one remaining chance for success.) or (The situation looked hopeless. There was one remaining chance for success.)
A common mistake made with the comma is to separate a dependent clause from an independent clause when they are joined with a conjunction. (I was told the situation looked hopeless but didn’t believe it.) Each clause must have a subject in order to need a comma before the conjunction.
According to The Elements of Style and The Chicago Manuel of Style, when listing three items, a comma is placed after the first and second items (Paper, pen, and writer). Some publishers omit the second comma, but they won’t fault you for not knowing their style. The important thing is to be consistent, so the editor can match the style sheet to your manuscript. Plus, if the list contains multiple words, it can be confusing if you don’t add the second comma. (His pets consisted of a long-haired cat, a short-haired dog, and a very noisy parrot.)
The exception to this rule concerns the name of businesses such as law firms which usually omit the last comma (Dewey, Sokum and Howe).
To further complicate things, if the list of items includes commas, they should be set off by semi-colons (The blank, white sheet of paper; the black, fine-line pen; and the ready, spirit-filled writer).