Position Three of the Comma Sutra in line editing addresses the commas which should occur between two, three, or more (though I wouldn’t recommend too many more) independent clauses in sentences. Usually, these independent clauses are connected by and, but, or or one of their siblings of nor, for, so, and yet. Or and nor bring up their own whole other issues – either, or and neither, nor – but that isn’t really about commas, so we’ll table the discussion for now.
Unless your sentence contains short independent clauses, you should use a comma before the and, etc. (the coordinating conjunctions) to, according to Diana Hacker in A Pocket Style Manual (5th Ed.) “tell readers that one independent clause has come to a close and that another is about to begin” (p. 58). As Ms. Hacker warns, this rule only applies to independent clauses.
Here are a couple of examples from Missing Emily: Croatian Life Letters:
Two short independent clauses where a comma is not necessary:
“I think she likes Mate but I am not worried.”
Longer independent clauses with a comma:
“I have not known what I should write to you, but I will try to answer the questions in your last letter.”
How do you know if your clauses are independent and whether they are short enough to omit the comma? You guessed it: read the work out loud. Read the clauses as their own sentences – such as “I think she likes Mate,” and, “I am not worried.” If they make sense as standalone sentences, they are independent. To determine if the length of the clause requires a comma, read the sentence out loud. If the meaning is clear, you can leave the comma out. If the meaning is confusing, or if by the time you finish the sentence, you are lost and don’t remember what the first part of the sentence said, you probably need a comma.
As with Positions One and Two of the Comma Sutra, this one has a degree of ambiguity, and it is open to varying subjective interpretations. What’s the bottom line? Seek out every coordinating conjunction and multi-clause sentence in your writing and question it. Make an informed, purposeful choice – comma or no comma – and go with it. Others may disagree, but you’ll have your thought-out reasons with which to defend yourself.
Source: Hacker, D. (2009). A Pocket Style Manual, 5th Ed. Bedford/St. Martin’s: Boston/New York.