When to use commas: 5 rules and examples
Dear Write Site is a new series that equips Athabasca University (AU) students with tips and tricks to improve their writing—whether it’s for an essay, research paper, or the next great novel. We feature advice from the Write Site, AU’s academic writing support services, with answers to student questions.
Dear Write Site,
My comma use, is, all over the place. I’m often called out for overusing commas, or not putting them in their proper place. Please help!
—Comma Me Confused
Dear Comma Me Confused,
Choosing when to use a comma can be overwhelming because it is governed by so many rules. We may be tempted to under-use commas because we aren’t confident that we know the rules, or we may be tempted to over-use them because we fear lengthy, run-on sentences. English is also relatively “comma-less” in comparison to other languages, adding another layer of confusion.
This article demystifies the top 5 comma rules so that you can feel more confident when writing. All 5 rules have something in common: separating information.
The word “comma” comes from the Greek word koptein, meaning “piece which is cut off.” Therefore, a comma “cuts off” a piece of information from the rest of the sentence. This makes reading and understanding easier when done correctly.
As you read, watch for examples of each rule in bold.
1. Use a comma to separate anything before an independent clause
An independent clause is a complete sentence on its own, and it must consist of two things:
- someone doing an action or being described (a subject) and;
- a verb, which is an action done by the subject (like write or read) or a word that allows the subject to be described (like is or seems).
If a piece of information comes before an independent clause, it should be “cut off” from the independent clause by a comma.
Notice the example in the previous sentence: the independent clause starts with “…it [subject] should be ‘cut off’ [verb] …” Therefore, a comma separates the dependent clause that precedes it. Again, notice the example in my previous sentence: the independent clause starts with “… a comma separates …” Thus, a comma separates the transition word “therefore” from the independent clause.
2. Use a comma to separate independent clauses joined by coordinating conjunctions
In some cases, you may link two related independent clauses to create a compound sentence. This can be done with a semi-colon in the middle; however, a comma with a coordinating conjunction is a common approach, so you should try it! Coordinating conjunctions are also called FANBOYS: For, And, Nor, But, Or, Yet, and So.
Remember that a comma alone is not strong enough between two independent clauses; this error is called a comma splice.
Instead, pair the comma with a coordinating conjunction, and you will achieve a grammatically correct compound sentence.
3. Use a comma to separate all non-essential information
Sometimes words, phrases, or clauses are non-essential, or unnecessary, to the understanding of the main idea of the sentence. These elements should be cut off from the rest of the sentence with opening and closing punctuation, like how parentheses work.
The first comma signals the start of the non-essential element; a second comma or period signals the end. The sentence thus provides pause before and after any excess describing information, improving readability. If you’re not sure whether information is non-essential, try reading the sentence without it.
The element is non-essential if it can be “cut off” and the meaning, whose basis is the remainder of the sentence, is still clear.
Tip: Non-essential clauses often begin with the relative pronoun “which.”
Related: Improve your writing with these proofreading strategies
4. Do not use a comma to separate essential information
What happens when you use a comma to cut off part of the sentence that is essential to its meaning? A reader who knows or intuitively understands rule No. 3 will perceive and treat that information as non-essential (as though it were contained within parentheses). If you tend to overuse commas, you may do this inadvertently, thus sacrificing valuable meaning.
Can you tell where the commas shouldn’t be in this, incorrect, sentence? The word “incorrect” is essential to the sentence’s meaning; otherwise, you may mistakenly think it exemplary!
Tip: Any clause that begins with the relative pronoun “that” is essential. Do not place a comma before “that!”
5. Do not use a comma to separate the subject from the verb
The person doing an action in a sentence should be as close as possible to the action they’re doing. This means that they shouldn’t be separated by a comma.
The simplest sentence requires only a subject and a verb, as in the answer to the question: “Are you leaving now?”—“I am.” Wouldn’t it be strange to say “I, am”? The action is being cut off completely from the person doing it, causing confusion.
It, makes much more sense to keep the subject and verb beside each other, unlike what I, have done in this incorrect example!
Exception: If you are adding non-essential information to the subject, such as an “appositive”—a noun or pronoun placed beside the subject to describe it—you must surround the phrase with an opening and closing comma.