Comma Rules: 10 Rules With Examples and Common Mistakes

Use a comma to separate items in a list, to join two complete sentences with a conjunction like and or but, after an introductory word or clause, and around information the sentence could do without. Don't use a comma alone to join two complete sentences, and don't put one between a subject and its verb. Those few rules cover most of the commas you'll ever write; the ten below cover nearly all the rest.

1. Use a comma before a conjunction that joins two complete sentences

When and, but, or, so, yet, for, or nor links two independent clauses (each could stand alone as a sentence), put a comma before the conjunction.

• The library closes at nine, but the study rooms stay open.
• I sent the draft on Monday, and my advisor replied the next day.

If the second part has no subject of its own, it isn't a complete sentence, so skip the comma:

• I sent the draft on Monday and waited for a reply. (no comma: "waited" shares the subject "I")

Very short clauses can go without the comma in informal writing ("I cooked and she cleaned"), but including it is never wrong.

2. Use commas between items in a list of three or more

• We need flour, eggs, and butter.
• She has lived in Denver, Austin, and Seattle.

The comma before the final and is the Oxford or serial comma. APA, MLA, and Chicago style use it; AP style, used by many newspapers, usually doesn't. It can prevent real confusion:

• I thanked my parents, Ana and Luis. (Are Ana and Luis your parents?)
• I thanked my parents, Ana, and Luis. (Four people, clearly.)

Whichever you choose, stay consistent within a document.

3. Use a comma after an introductory word, phrase, or clause

When something comes before the main subject and verb, a comma usually marks where the main sentence starts.

• However, the results were mixed.
• After the meeting ended, we went to lunch.
• To be honest, I'd prefer the first option.
• If you finish early, check your citations.

With a short introductory phrase (about four words or fewer), the comma is optional: "In 2020 the rules changed" is fine. After an introductory clause with its own verb, like "If you finish early," include it.

4. Use commas around nonessential information

If a word, phrase, or clause adds extra detail and the sentence would still mean the same thing without it, set it off with commas, one on each side.

• My sister, who lives in Toronto, is visiting next week.
• The report, written in a hurry, had several errors.
• Dr. Patel, our new advisor, starts in January.

If the information is essential because it identifies which one you mean, don't use commas:

• Students who submit late lose ten points. (Which students? Only the late ones.)
• Students, who submit late, lose ten points. (Now it says all students submit late.)

The same idea explains a common American English convention: use which with commas for nonessential clauses and that without commas for essential ones. "The car, which is red, needs repairs" versus "The car that is red needs repairs."

5. Use a comma between coordinate adjectives

When two or more adjectives describe a noun equally, separate them with a comma. Two quick tests: you can put and between them, or reverse their order, and the phrase still sounds natural.

• a long, difficult exam ("long and difficult" works, so use a comma)
• a cold, rainy morning

If the tests fail, there's no comma:

• a bright red car ("bright and red" changes the meaning)
• three old wooden chairs ("wooden old chairs" sounds wrong)

6. Use a comma when you address someone directly

• Maria, can you send me the file?
• Thanks for the update, Sam.
• I think, professor, that the deadline was Friday.

This one matters more than it looks: "Let's eat, Grandma" and "Let's eat Grandma" are very different invitations. It's also why email greetings are written "Hi Jordan," with the comma after the name. Strictly, "Hi, Jordan," follows the direct-address rule, but most people drop the first comma in emails and nobody minds.

7. Use commas in dates, places, and numbers

• On July 4, 1776, the declaration was adopted. (In American style, a comma follows the year when the sentence continues.)
• She moved to Portland, Oregon, in 2019. (The state is set off on both sides.)
• The town has about 12,000 residents.

No comma is needed with just a month and year ("in March 2025") or with the day-month-year order ("4 July 1776").

8. Use a comma to introduce or close a direct quotation

• She said, "We'll start at noon."
• "We'll start at noon," she said.

Notice the comma goes inside the closing quotation mark in American English. Skip the comma when the quotation is woven into your sentence: The manual calls the step "optional but recommended." Our guide on when to use quotation marks covers the rest of the rules for quoting.

9. Use a comma before a contrasting element or a tag question

• The meeting is on Thursday, not Friday.
• It was the design, rather than the price, that sold the client.
• You submitted the form, didn't you?

10. Don't use a comma to join two complete sentences

This is the most common comma error, called a comma splice:

• ✗ The deadline moved, we have an extra week.

A comma alone is too weak to hold two sentences together. Use a period, a semicolon, or a comma plus a conjunction:

• ✓ The deadline moved. We have an extra week.
• ✓ The deadline moved; we have an extra week.
• ✓ The deadline moved, so we have an extra week.

Words like however and therefore don't fix a splice on their own: write "The deadline moved; however, we're still behind." Our guide to run-on sentences and comma splices goes through all four fixes in detail.

Where commas don't belong

Most comma errors are extra commas, not missing ones. Leave them out in these spots:

• Between a subject and its verb: ✗ "The students who arrived late, missed the quiz."
• Between a verb and its object: ✗ "She explained, that the room had changed."
• Before an essential clause: ✗ "The book, that you lent me, was great."
• After such as or like: ✗ "Bring supplies such as, pens and paper."
• Before because in most sentences: "I left early because I was tired" needs none.

If you want an emphatic break rather than a comma, a dash may be the better tool. See how to use a dash for when an em dash beats a comma.

Check your commas

Reading aloud catches some comma problems, but not all, because commas follow structure rather than breath. When you revise, look for every and, but, and so and ask whether a full sentence sits on each side. Then look at your opening phrases and your who and which clauses.

ZeroTell's Grammar Checker flags punctuation problems, including comma splices and run-on sentences, and can give a short reason for each change so you can see which rule applies. If a long sentence needs restructuring rather than one more comma, try the Sentence Rewriter.

Grammar CheckerFix grammar and phrasing

More grammar guides

Commas are one piece of clear writing. These guides cover the other mix-ups that editors fix most:

• Run-on sentences and comma splices: four ways to fix them
• Affect vs. effect: the rule, the exceptions, and a memory trick
• Who vs. whom: the he/him test
• Its vs. it's and your vs. you're: when the apostrophe belongs
• Active vs. passive voice: when each one works

Key takeaways

• Comma + conjunction joins two complete sentences; a comma alone doesn't.
• Use commas after introductions, between list items, and around nonessential details.
• Essential information gets no commas.
• Never separate a subject from its verb with a single comma.
• Pick a style for the Oxford comma and use it consistently.