The rules
| 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. |
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.
- Find the introduction: After the game ended. Add a comma after it.
- 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.
- 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.
- Is which is the largest of the Great Lakes needed to know which lake? No, Lake Superior already names it. So it is extra.
- Extra information needs a comma before it and after it.
- 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
- Try and between the words: a cold and windy morning sounds right, so cold and windy are equal. Use a comma.
- 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. |
| 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
Which sentence is punctuated correctly?
- I wanted to go to the concert but, the tickets sold out.
- I wanted to go to the concert, but the tickets sold out.
- I wanted to go to the concert, the tickets sold out.
- I wanted, to go to the concert but the tickets sold out.
Show answer
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.
Which sentence is punctuated correctly?
- When, the bell rang students rushed out.
- When the bell rang students, rushed out.
- When the bell rang, students rushed out.
- When the bell rang; students rushed out.
Show answer
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.
Which sentence punctuates the extra information correctly?
- Mount Rainier which is an active volcano, is in Washington.
- Mount Rainier, which is an active volcano, is in Washington.
- Mount Rainier, which is an active volcano is in Washington.
- Mount Rainier which, is an active volcano is in Washington.
Show answer
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.
Which sentence has a comma error?
- Before lunch, we finished the lab.
- The test was long, but it was fair.
- We bought apples and pears at the market.
- The students who finished the test early, went to the library.
Show answer
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.
Which sentence is punctuated correctly?
- It was a cold, windy morning.
- It was a cold windy, morning.
- It was a, cold windy morning.
- It was, a cold windy morning.
Show answer
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, accessed October 1, 2026
- Purdue OWL, Commas vs. Semicolons, accessed October 1, 2026
