A comma goes before words like and, but, and or—except when it doesn’t.
|Have you made the appropriate sacrifices, or will you be leaving our fate up to luck or chance?|
When And Needs a Comma
It’s traditional to put a comma before words such as and, but, or, so, and yet (a.k.a. coordinating conjunctions) when they’re joining two independent clauses. Independent clauses are clauses that could stand on their own as sentences if they wanted to. Think of them as train cars that could travel by themselves but have been hitched together to form one sentence.
- I’ve trained the squirrels outside my window to do backflips and pyramids, and we’ll be taking our act on the road next week.
- He bought the jester’s costume and the floppy hat, but I can’t help feeling his heart isn’t in it.
- Have you made the appropriate sacrifices, or will you be leaving our fate up to luck or chance?
If the sentence is quite short or its clauses are closely connected, you may leave out the comma.
- Mom played and Dad sang.
- The pirate queen unsheathed her cutlass and I knew it was over.
When But Doesn’t Need a Comma
Conjunctions don’t always join independent clauses—and they don’t always need commas.
- Clyde wanted to go to the orgy but couldn’t find any whipping cream.
“Couldn’t find any whipping cream” is not an independent clause: it lacks a subject and would never make it on its own as a complete sentence. Its subject, Clyde, is doing double duty for two verbs, wanted and couldn’t. (This is called a compound predicate, if you want to get fancy.)
- Clyde wanted to go to the orgy, but he couldn’t find any whipping cream.
Now each clause has both a subject and a verb, so our but can take a comma.
Similarly, when a single verb is shared between two subjects (called a compound subject), you don’t need a comma before the conjunction joining them together.
- Crawling up the side of a castle and crawling down the side of a castle are different levels of creepy.
As you can see, this rule applies to other conjunctions besides but.
- I knew the full moon was coming so didn’t bother to shave.
- He tried valiantly yet failed miserably.
|I knew the full moon was coming so didn't bother to shave.|
A comma might be found before an or or an and if it’s one of a pair enclosing an aside, or parenthetical phrase. These are phrases that are not essential to the sentence and could just as well be set between a pair of parentheses or dashes.
- Sook-Yin’s doppelgänger, or double, had been lurking in the lobby all week.
- If you know what I mean, and I think you do, give me a wink.
But with Not or Not Only
Constructions like “not only…but…” don’t take commas before the but.
- The cowboy was impressed not only by her hair but also her horns.
However, you may use a comma for emphasis if you choose.
- It would be not only his first trip to the Sierras, but his last.
|The cowboy was impressed not only by her hair but also her horns.|
Discretionary Commas: When to Break the Rules
Sometimes punctuation is as much an art as a science—particularly when it comes to commas. As in the case of the pirate queen earlier, it’s okay to ignore the “put a comma before a coordinating conjunction when it joins two independent clauses” rule if the clauses are short or tightly connected. You may even break up a compound predicate if you want to highlight the contrast between its verbs.
- The adventuress declared her eternal devotion to Alberto, yet left him to perish in the swamp.
Your most important consideration as a writer is clarity. An unconventional comma placement is sometimes necessary to keep readers from misunderstanding you. Notice how commas change the meanings of the following sentences:
- Jasika recognized the supervillain as he leaned out the porthole, and pointed an accusing finger.
- If you’re ready, and I think you’re up to it, you can dance the funky chicken at my bat mitzvah.
In creative writing and informal prose, commas are often discarded as a matter of style. It’s up to you as a writer to decide on the appropriate amount of punctuation for your piece. But before you can successfully flout the rules, you must understand exactly what you’re doing, and why.