You’re proofreading a client email five minutes before it goes out, and you’ve reached the sentence where you can’t tell whether it needs a comma. Most writers in that spot add one at random. A handful of comma rules cover nearly every ordinary sentence, and learning five of them removes most of the guesswork.
The five comma rules below come from university writing-centre guidance in the US, mainly Purdue’s Online Writing Lab and the University of North Carolina Writing Center. The examples are original, so you can copy the patterns without copying anyone’s sentences.
The five comma rules at a glance
Style guides disagree on small points, but the following five rules are common ground. Learn the pattern first, then the exceptions.
| Rule | Use a comma when… | Example |
|---|---|---|
| 1. Introductory words | a word, phrase or clause comes before the main clause | After the call, we updated the schedule. |
| 2. Series | you list three or more items | We tested the headline, the photo and the button. |
| 3. Joined sentences | and, but, or, nor, for, so or yet joins two full sentences | The draft is done, but the images are missing. |
| 4. Extra information | a phrase adds detail you could remove (use a pair of commas) | The report, published in March, drew little attention. |
| 5. Separate adjectives | two adjectives each describe the noun on their own | She wrote a clear, short summary. |
Rule 1: Put a comma after an introductory element
Purdue’s guide says to “use commas after introductory a) clauses, b) phrases, or c) words that come before the main clause.” In practice that covers openers such as “However,” “Last week,” and “If the file is ready,”.
- Word: Finally, the invoice arrived.
- Phrase: After a long delay, the invoice arrived.
- Clause: When the invoice arrived, we paid it.
The UNC Writing Center explains the reason: the comma “lets the reader know that the main subject and main verb of the sentence come later.” Very short openers such as “Today we met” often go without one, and a comma after a short opener is usually acceptable, so choose one habit and apply it consistently.
Rule 2: Separate items in a series, and pick a side on the last comma
Purdue’s rule is short: “Use commas to separate three or more words, phrases, or clauses written in a series.” So: The kit includes a brush, a tin and a cloth.
The Oxford comma
The comma before the final “and” is the serial or Oxford comma. UNC calls it “optional, but it is never wrong,” and says to keep it when the items are long or complicated. Pick one habit per document, and your house style guide, if you have one, settles the argument.
The comma earns its place when a list could be misread. Compare these two:
- We dedicated the project to our mentors, a journalist and a poet. This reads as though the mentors are a journalist and a poet.
- We dedicated the project to our mentors, a journalist, and a poet. Now there are three groups of people.
Rule 3: Put a comma before a conjunction that joins two full sentences
A full sentence, or independent clause, has its own subject and verb. When one of the seven joining words (for, and, nor, but, or, yet, so) links two of them, the comma goes before the joining word. UNC’s example: They smell really bad, so they will be easy to find.
Test it by reading the words after the joining word on their own. If they form a complete sentence, add the comma: The team shipped the update, and the support inbox went quiet. If they don’t, leave it out: The team shipped the update and went home. UNC makes the same point with its hamster example, where the words after “and” are not a complete thought.
Test the words after “and” or “but” on their own. If they can stand alone as a sentence, the comma goes before the joining word.
Rule 4: Wrap extra information in a pair of commas
Some phrases add detail without changing which thing you mean. Purdue says to “use a pair of commas in the middle of a sentence to set off clauses, phrases, and words that are not essential”. UNC offers a quick check: remove the phrase, and if the sentence still reads clearly, it needs commas on both sides.
- The warehouse, which opened in 2019, handles all returns. Remove the middle and the sentence survives.
- The warehouse that opened in 2019 handles all returns. The phrase identifies which warehouse, so there are no commas.
The common error is opening the pair and forgetting to close it. If you can find a comma before the extra detail, look for its partner afterwards.
Rule 5: Separate adjectives that work independently
Purdue’s rule: “Use commas to separate two or more coordinate adjectives that describe the same noun.” It also warns: “Be sure never to add an extra comma between the final adjective and the noun itself”.
A practical test, which is advice rather than an official rule, is to put “and” between the adjectives or swap their order. A clear and short summary works, and so does a short, clear summary, so the comma stays. A small wooden desk fails both tests (nobody says “a wooden small desk”), so no comma.
The mistakes that cost the most
The comma splice
A comma splice joins two complete sentences with a comma alone, as in The draft is ready, we can publish. UNC describes it as a comma used alone between two complete thoughts. There are three standard fixes:
- Split it into two sentences with a full stop.
- Add a joining word after the comma: The draft is ready, so we can publish.
- Rewrite so one part depends on the other: We can publish because the draft is ready.
One comma between the subject and the verb
A single comma that separates the subject from its verb is wrong in standard writing: The staff who stayed late, finished the report. Remove the comma. A pair of commas around extra information (rule 4) is fine, because the second comma closes the interruption.
Placing commas where you pause
Many writers were taught to add a comma wherever they would breathe. UNC rejects that directly: “Where you pause or breathe in a sentence does not reliably indicate where a comma belongs.” Reading aloud can still help you hear a muddled sentence, but the five rules above are the better guide for placing the comma.
Example: one paragraph, five fixes
Here is a paragraph before and after. It is invented for practice.
Before: After the launch we reviewed the results and the numbers were lower than expected. Our new smaller team which started in June handled the support queue. The queue held tickets about billing login and refunds.
After: After the launch, we reviewed the results, and the numbers were lower than expected. Our new, smaller team, which started in June, handled the support queue. The queue held tickets about billing, login and refunds.
Count the changes: a comma after the opener (rule 1), a comma before “and” because two full sentences are joined (rule 3), commas around “which started in June” (rule 4), a comma between two independent adjectives (rule 5), and commas in the list (rule 2). That’s six commas added to a three-sentence paragraph, and each one has a reason.
A 60-second comma check before you hit send
- Does the sentence open with a word, phrase or clause before the main clause? Add a comma after it.
- Is there a list of three or more? Separate the items, and use your final comma consistently.
- Does a joining word link two sentences that could stand alone? Put the comma before it.
- Could you delete a phrase and keep the sentence? Wrap it in a pair of commas.
- Do two adjectives each describe the noun? Separate them, but never put a comma after the last one.
- Is there one comma between the subject and the verb? Delete it.
- Are two full sentences joined by a comma alone? Fix the splice.
The takeaway
Five rules handle most comma decisions: introductory elements, series, joined sentences, extra information and independent adjectives. Add the splice check and the subject-verb check, and nearly everything that remains is a matter of house style. When two guides disagree, as they do over the final comma in a list, choose one and apply it every time.
