7 Comma Rules Every Writer Should Know

In the beginning of 2018, one missing comma in a Maine state law cost Oakhurst Dairy five million dollars.

The absence of the Oxford comma in the labor law made two overtime-exempt tasks, packing and distributing food products, read like one activity, so any worker who distributed but didn’t pack any food products or vice versa sued the dairy company for their unsettled overtime pay. The workers won. 

As you can see, commas might seem insignificant in the grand scheme of things, but even one tiny error can cost you millions of dollars.

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As a writer, correct comma usage is especially important because writing a piece packed full of comma errors could damage your professional reputation.

Commas are tiny yet potent details you must pay a laser-like attention to, so we decided to create a list of the seven comma rules every writer should know. And hopefully, refreshing your memory on the correct way to use commas will help you eliminate any chance of embarrassment from making these discreetly dangerous grammatical errors.

7 comma rules every writer should know

  1. You can only connect two complete sentences with a comma if there’s a coordinating conjunction (for, and, nor, but, or, yet, so), which are also called FANBOYS, between them.
  2. Use a comma to link an incomplete sentence with a complete sentence.
  3. Use the Oxford comma.
  4. Use a comma between two coordinate adjectives that describe the same noun.
  5. Use a comma to highlight additional, non-essential information about a noun.
  6. Make sure you place commas correctly in quotes.
  7. Put a comma before “while” to contrast two things.

1. You can only connect two complete sentences with a comma if there’s a coordinating conjunction (For, And, Nor, But, Or, Yet, So, or FANBOYS) between them. 

A complete sentence has a subject, verb, expresses a complete thought, and it can be its own sentence. For example, “We found a lost dog.” and “We took her home.” are both complete sentences.

But if you place a comma between the two, like in the sentence “We found a lost dog, we took her home.” you end up writing a comma splice.

To fix this, you need to insert a coordinating conjunction or one of the FANBOYS after the comma, like this: “We found a lost dog, so we took her home.” This is how you correctly use a comma to connect these two complete sentences.

2. Use a comma to link an incomplete sentence with a complete sentence.

An incomplete sentence can have a subject and verb, but it doesn’t express a complete thought, so it can’t be its own sentence. For instance, the sentence “Even though I tell my friends I hate Nickelback” doesn’t express a complete thought and can’t stand alone. Therefore, it’s an incomplete sentence.

To make it complete, you need to add a comma and a complete sentence to the clause, like “I went to their concert”. When you do this, you’ve used a comma to connect an incomplete sentence and a complete sentence together, like this: “Even though I tell my friends I hate rock music, I went to Nickelback’s concert.”

It also doesn’t matter whether your incomplete or complete sentence comes first. For example, “I went to Nickelback’s concert, even though I tell my friends I hate rock music.” is also the correct use of a comma.

3. Use the Oxford comma.

The Oxford comma might be one of the most controversial topics in the history of grammar. For years, people have argued whether it’s necessary or not, but a lot of publications, including HubSpot’s blog, believe it’s crucial for clarifying sentences that separate items in a list.

For instance, the sentence, “Today, I watched a documentary about Babe Ruth, a New York City mobster, and the founder of New York-Style pizza.”, expresses that I watched a documentary about three famous people who lived in New York City.

If you neglect the Oxford comma in this sentence, though, you end up with something like, “Today, I watched a documentary about Babe Ruth, a New York City mobster and the founder of New York-style Pizza.”

Unless Babe Ruth also lived a secret life of crime and thin-crust pizza, you can see how the absence of the Oxford comma in this sentence could confuse readers.

4. Use a comma between two coordinate adjectives that describe the same noun.

Adjectives are considered coordinate when your sentence still makes sense after swapping the adjectives and placing an “and” between the two.

For instance, you can rewrite “I saw a tiny, adorable puppy when I drove to the store” as “I saw a tiny and adorable puppy when I drove to the store”. And since the adjectives “tiny” and “adorable” are interchangeable and you can place an “and” between them without muddling the sentence’s meaning, the adjectives are coordinate, so you should place a comma between the two.

Sentences with non-coordinate adjectives, like “I just ate some pungent chicken soup.” don’t need a comma. In this example, “chicken” is an adjunct noun, which means it acts like an adjective describing the noun “soup”. But since “chicken” is technically still a noun, and not an adjective, you don’t need to put a comma between “pungent” and “chicken”.

5. Use commas to highlight additional, non-essential information about a noun.

In the sentence, “I saw tons of Ospreys, also known as Sea Hawks, when I went to the Everglades.”, the words isolated by the commas give additional information about the noun, Ospreys, but they’re not essential for the sentence to make sense.

For example, you could write “I saw a ton of Ospreys when I went to the Everglades” and the sentence still makes sense. Consequently, including any additional, non-essential information about a noun requires you to isolate it from the rest of the sentence with commas.

If any additional, non-essential information is at the end of your sentence, you need to put a comma before it, like, “When I went to the Everglades, I saw a ton of Ospreys, also known as Sea Hawks.”

6. Make sure you place commas correctly in quotes.

When you write a sentence with a quote, the placement of a comma depends on whether the attribution is before or after the quote.

For instance, when the attribution is before the quote, like in the sentence “The cook said, ‘I need more cheese.’”, you need to place the comma outside of the quotation marks.

When the attribution is after the quote, like in the sentence “‘I need more cheese,’ said the cook.”, you need to place the comma inside the quotation marks.

7. Put a comma before “while” to contrast two things.

Putting a comma before “while” is the equivalent of saying “whereas”. So if you want to contrast two things, like “I like working on more analytical projects, while my brother likes working on more creative projects”, you need to put a comma before “while”.

Leaving the comma out before “while” will describe two events that occur simultaneously. In the sentence “I ran to the drug store to get ice while Greg drove to my house to get our cooler”, I’m expressing that Greg and I went to different locations to grab ice and a cooler at the same time.

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