Last month, I started covering rules on commas. I’m continuing covering more rules on commas here. (Yes, there are a lot of them!)
As I mentioned last month, it’s important to remember that the overall goal of the comma is to increase readability. The comma rules I’m covering are based on Chicago Manual of Style (CMOS), which is the style guide used for fiction. Other style guides may have slightly different rules. However, they’re mostly the same. Your copyeditor or proofreader can change any minor differences to conform with the appropriate style guide.
Joining Two Independent Clauses
You should nearly always use a comma when joining two independent clauses with a conjunction (e.g., and, but, or). Independent clauses are two independent thoughts that could stand as sentences on their own; each clause has its own subject and verb.
I like Thanksgiving, but Christmas is my favorite holiday.
In this example, “I like Thanksgiving” and “Christmas is my favorite holiday” could both be sentences on their own. They’re joined by the conjunction “but” to make clearer the relationship between the two clauses. Therefore, a comma is used before the conjunction to separate the two clauses. This rule also applies for less common conjunctions:
I like Thanksgiving, though Christmas is my favorite holiday.
There are two exceptions to this rule, and they both should be rare and for stylistic purposes only. First, you may have a character who speaks a sentence without a break, and you want the reader to read it that way:
Chris said, “I love you but I don’t know if you love me and it’s killing me not knowing.”
Can’t you imagine Chris getting that sentence out in one breath, without pause, because it took all of his courage to say it, and he wants to get it over with? That’s the power of punctuation and occasionally breaking the rules. If you do it all the time, though, readers will become immune to it and will think the lack of commas are accidental rather than purposeful.
The other time you may break the rule is to have a comma splice. It’s not quite the same situation as above. A comma splice is where you have a comma to separate the two independent clauses, but you don’t include the conjunction. This is also common in dialogue, though you can use it elsewhere too.
”We need to get going, we told the babysitter we’d be back by ten,” Marcy said.
“We need to get going” and “we told the babysitter we’d be back by ten” are both independent clauses, but there’s no conjunction joining them. Why would you do this? Because people often speak this way, and you want your dialogue to be realistic. The speaker didn’t pause long enough for there to be a full stop period or even a semicolon. You want your readers to read it as a very brief pause. As with the previous exception, don’t overuse it, or readers will think you don’t know the rules rather than that you’re doing it purposefully.
Commas with Dependent Clauses
When using a dependent clause (not a full sentence) before an independent clause, the dependent clause should be set off by a comma.
Before it starts raining, put on your raincoat.
“Before it starts raining” is not a sentence on its own. It’s dependent on the independent clause “put on your raincoat,” which could be a sentence on its own. Additionally, the dependent clause precedes the independent clause. Therefore, the clause “before it starts raining” needs a comma after it for clarity.
When the dependent clause comes after the independent clause, it’s not as straightforward, unfortunately. The rule depends on whether the dependent clause is restrictive (essential for fully understanding the independent clause) or unrestrictive (could be left out without significantly altering the meaning of the independent clause).
I’ll sign the contract if you change the deadline.
The above is an example of a restrictive dependent clause. “I’ll sign the contract” is the independent clause in this sentence and “if you change the deadline” is the dependent clause. The dependent clause is restrictive in this case because the person will not sign the contract unless the deadline changes. Therefore, the dependent clause is essential to the meaning of the sentence. Since it is essential, it does not get a comma.
I’ll sign the contract now, if you please.
On the other hand, this example has an unrestrictive dependent clause. “If you please” does not change the meaning of the independent clause “I’ll sign the contract.” It could be left out and wouldn’t change the message. Since it is not essential to understanding, it gets a comma.
The rules are the same for restrictive or unrestrictive clauses, words, or phrases that appear in the middle of the sentence: if it’s restrictive, then don’t use a comma; if it’s unrestrictive, use a comma.
My sister Marie cared for me after surgery.
My book The Joy of Editing was very successful.
The movie we saw that starred Wilson Cooper won an Academy Award.
If the speaker has more than one sister, then it’s necessary information to know the sister’s name to identify which sister is being spoken about and therefore does not get a comma. Similarly, if the speaker has written multiple books, then the title is necessary information for knowing which one she’s talking about. Finally, if the speaker is talking to someone with whom she’s seen multiple movies, then it’s necessary to know she’s talking about the one that starred Wilson Cooper. All of these are restrictive clauses, words, or phrases that appear in the middle of a sentence and are necessary for full understanding of the sentence. Therefore, none of them get commas.
My husband, Andrew, cared for me after my surgery.
My first book, The Joy of Editing, was very successful.
The movie we saw last Friday, which starred Wilson Cooper, won an Academy Award.
Unlike in the restrictive example, the speaker most likely only has one husband; therefore, the husband’s name is extraneous information that does not aid in understanding the sentence. Similarly, by adding that it was the speaker’s first book that was successful, the title becomes unnecessary; the reader would already know which book was successful without the title. In the last example, by adding that the movie was seen last Friday, it’s probably enough information to know which movie was meant without including who starred in it. All of these are unrestrictive dependent clauses, words, or phrases that don’t change the core understanding of the sentence. Therefore, they get commas both before and after them to set them off from the surrounding sentence.
Now that you understand restrictive and unrestrictive dependent clauses, let’s go one step further. When you’re deciding whether to use that or which for a dependent clause, that should be used for restrictive dependent clauses while which should be used for an unrestrictive dependent clause.
I prefer to eat vegetables that are in season.
“That are in season” is essential to the meaning of the sentence. Therefore, that is used, and no comma is needed.
I prefer summer squash, which is not in season right now, over winter squash.
“Which is not in season right now” is not essential to understanding that the speaker prefers summer squash over winter squash. Accordingly, which is used, and a comma is required.
Commas aren’t your thing? I can help!