Commas Used in Compound Sentences.
Comma confusion and conjunctions seem to shadow one another. For now let’s address coordinating conjunctions, specifically the “Big Three” which are the most common—and, but, or.
These conjunctions join “words or groups of words of equal grammatical rank” (Chicago Manual of Style, 5.181). That is, they join noun with noun (boy and his dog), verb with verb (typed the letter and printed it out), prepositional phrase with prepositional phrase (in the yard or on the sidewalk), and clause with clause (she ran but he stopped).
Where does the comma fit in with these? As discussed in Punctuation Pitfalls – The Comma, Part 1, Chicago recommends using a comma before the conjunction in a list of three or more items, but there is another occasion unique to clauses.
When two clauses—groups of words with a subject and verb—could stand independently as a sentence, they are called independent clauses (grammarians aren’t known for creativity – 😉 ). A sentence with two or more independent clauses—in other words, a compound sentence—will employ a comma before the conjunction.
Here are a couple examples from Curse of the Spider King by Wayne Thomas Batson and Christopher Hopper:
- Johnny and Autumn couldn’t see what she was doing, but they heard a muted thump.
- Oh, they never said it out right, but they didn’t have to.
- The rain came down in earnest now, and Jimmy pulled his slicker tight around himself.
The clue here is to avoid confusing clauses with compound elements (more than one phrase or multiple verbs and their attending describers). And of course there is an exception—a “your choice” exception.
To avoid the confusion, read the material prior to the conjunction and see if it could stand alone, then read the material after the conjunction and see if it also could stand alone. If both are grammatically independent, the sentence is compound. Use a comma before the conjunction.
The exception comes into play when the clauses are short and closely related. Chicago says, in those instances the comma may be omitted. So, for example, in a sentence such as Mom baked the turkey and her sister made pies, the comma before the conjunction may be left out. Or not. You get to choose.
My suggestion? Until you feel fairly comfortable identifying compound sentences apart from sentences with compound elements, use the comma. It will force you to think about the sentence construction.