# Comma rules

Canonical: https://duckyhelper.com/learn/grammar/commas/
Updated: 2026-10-01

Use a comma before and, but, or, so, yet, for or nor when it joins two complete sentences; after an introductory word, phrase or clause; between items in a list; around extra information the sentence could lose; and between two describing words that each describe the same noun. Never use a comma alone to join two sentences, and never put one between a subject and its verb.

## The rules

**Six comma rules with examples**

| Rule | Wrong | Right | Why |
| --- | --- | --- | --- |
| Before *and, but, or, so, yet, for, nor* joining two sentences | *I studied all night but I still felt nervous.* | *I studied all night, but I still felt nervous.* | Both sides are complete sentences, so a comma goes before the joining word. |
| After an introduction | *After the game ended the team went for pizza.* | *After the game ended, the team went for pizza.* | The comma shows where the introduction stops and the main sentence starts. |
| Between items in a list | *We need eggs milk and bread.* | *We need eggs, milk, and bread.* | Commas separate the items. The comma before *and* is the serial comma (see the note below). |
| Around extra information | *Mount Rainier which is an active volcano, is in Washington.* | *Mount Rainier, which is an active volcano, is in Washington.* | Extra information gets a comma on both sides, like a pair of handles. |
| Between equal describing words | *It was a long boring movie.* | *It was a long, boring movie.* | If you can swap them or put *and* between them, use a comma. |
| Not between subject and verb | *The students who finished early, went outside.* | *The students who finished early went outside.* | However long the subject is, no single comma separates it from its verb. |

> **Note: The serial comma**
>
> The comma before *and* in a list of three or more is called the serial or Oxford comma. Chicago, MLA and APA style use it, and Purdue OWL calls it a general academic convention. AP style, used by many newspapers, leaves it out unless it is needed for clarity. Follow the style your teacher asks for and be consistent.

## Essential or extra?

Information that tells you *which one* is essential, so it takes no commas: *The phone that I dropped still works.* Information that just adds a detail is extra, so it takes commas: *My phone, which I dropped yesterday, still works.* A quick test: cover the words. If the sentence still says exactly who or what you mean, they were extra.

## Worked examples

**Example 1: fix a sentence with two missing commas**

Problem: *After the game ended the team went to get pizza but the bus was late.*

1. Find the introduction: *After the game ended*. Add a comma after it.
2. Look for two complete sentences joined by a joining word: *the team went to get pizza* and *the bus was late*, joined by *but*. Add a comma before *but*.
3. Check the subject and verb (*team went*): nothing should separate them, and nothing does.

Answer: *After the game ended, the team went to get pizza, but the bus was late.*

**Example 2: commas around extra information**

Problem: *Lake Superior which is the largest of the Great Lakes borders three states.*

1. Is *which is the largest of the Great Lakes* needed to know which lake? No, *Lake Superior* already names it. So it is extra.
2. Extra information needs a comma before it and after it.
3. Read it without the extra part: *Lake Superior borders three states.* It still works, which confirms the commas.

Answer: *Lake Superior, which is the largest of the Great Lakes, borders three states.*

**Example 3: equal describing words**

Problem: Do these need a comma? *a cold windy morning* and *a bright red jacket*

1. Try *and* between the words: *a cold and windy morning* sounds right, so *cold* and *windy* are equal. Use a comma.
2. *A bright and red jacket* sounds wrong, because *bright* describes *red*, not the jacket. No comma.

Answer: *a cold, windy morning*; *a bright red jacket*

## Common mistakes and fixes

| Mistake | Fix |
| --- | --- |
| Joining two sentences with only a comma (a comma splice) | Add *and*, *but* or *so* after the comma, or use a period or a [semicolon](https://duckyhelper.com/learn/grammar/semicolons/). |
| Putting a comma wherever you would pause | Pauses are not a rule. Check the six rules above instead. |
| Opening extra information with a comma and forgetting the closing one | Extra information needs a pair. Find where it ends and add the second comma. |
| Putting the comma after *but* instead of before it | The comma goes before the joining word: *, but*. |
| Adding a comma before *because* by habit | Usually no comma when the *because* part comes at the end: *I stayed home because I was sick.* |

## Practice

**Five comma questions**

1. Which sentence is punctuated correctly?
   A. I wanted to go to the concert but, the tickets sold out.
   B. I wanted to go to the concert, but the tickets sold out.
   C. I wanted to go to the concert, the tickets sold out.
   D. I wanted, to go to the concert but the tickets sold out.

   Answer: I wanted to go to the concert, but the tickets sold out.. Two complete sentences joined by *but* take a comma before *but*. *but,* puts the comma after the joining word, *concert, the tickets* is a comma splice, and *I wanted, to go* splits a verb from its object.

2. Which sentence is punctuated correctly?
   A. When, the bell rang students rushed out.
   B. When the bell rang students, rushed out.
   C. When the bell rang, students rushed out.
   D. When the bell rang; students rushed out.

   Answer: When the bell rang, students rushed out.. The comma goes after the introductory clause *When the bell rang*. *rang students, rushed* and *When, the bell* put commas in the wrong places, and a semicolon cannot follow a dependent clause.

3. Which sentence punctuates the extra information correctly?
   A. Mount Rainier which is an active volcano, is in Washington.
   B. Mount Rainier, which is an active volcano, is in Washington.
   C. Mount Rainier, which is an active volcano is in Washington.
   D. Mount Rainier which, is an active volcano is in Washington.

   Answer: Mount Rainier, which is an active volcano, is in Washington.. *Which is an active volcano* is extra, since the name already tells you which mountain. Extra information needs a comma on both sides. Two choices have only half of the pair, and *which, is* puts a comma inside the clause.

4. Which sentence has a comma error?
   A. Before lunch, we finished the lab.
   B. The test was long, but it was fair.
   C. We bought apples and pears at the market.
   D. The students who finished the test early, went to the library.

   Answer: The students who finished the test early, went to the library.. The subject is *The students who finished the test early*, and the verb is *went*. A single comma cannot separate a subject from its verb. The other three sentences follow the rules.

5. Which sentence is punctuated correctly?
   A. It was a cold, windy morning.
   B. It was a cold windy, morning.
   C. It was a, cold windy morning.
   D. It was, a cold windy morning.

   Answer: It was a cold, windy morning.. *Cold* and *windy* both describe the morning and can be joined by *and*, so a comma goes between them. A comma never goes between the last describing word and the noun, or after *a* or *was*.

## Frequently asked questions

### Do I need a comma before and in a list?

It depends on the style you follow. Chicago, MLA and APA use the serial comma (*eggs, milk, and bread*). AP style, used by many newspapers, leaves it out unless the sentence would be unclear. Most school writing follows MLA or APA, so using it is usually safe. Ask your teacher and stay consistent.

### Is a comma the same as a pause?

Not reliably. Sometimes you pause where a comma goes, but you also pause in places that need no comma, like after a long subject. Use the rules: joining two sentences with *and* or *but*, after introductions, in lists, around extra information, and between equal describing words.

### Do I put a comma before because?

Usually not when the *because* part comes at the end: *We left early because it was snowing.* When the *because* part comes first, it is an introduction, so add a comma after it: *Because it was snowing, we left early.*

## Sources

- [Purdue OWL, Commas: Quick Rules](https://owl.purdue.edu/owl/general_writing/punctuation/commas/index.html), accessed 2026-10-01
- [Purdue OWL, Commas vs. Semicolons](https://owl.purdue.edu/owl/general_writing/punctuation/commas/commas_vs_semicolons.html), accessed 2026-10-01

## Related

- [When to use a semicolon](https://duckyhelper.com/learn/grammar/semicolons/)
- [Run-on sentences, comma splices and fragments](https://duckyhelper.com/learn/grammar/run-on-sentences-and-fragments/)
- [SAT punctuation: Boundaries questions](https://duckyhelper.com/learn/sat-reading-writing/boundaries-punctuation/)
- [Grammar study guides](https://duckyhelper.com/learn/grammar/)

## Try asking Ducky

- "Read my essay and point out every comma splice, but let me fix them myself."
- "Is this information essential or extra? Help me decide if it needs commas."
- "Quiz me on comma rules with sentences from my own paragraph."

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