Comma Before Which, Who, Where or When? Rules and Examples

Put a comma before which, who, where or when if the clause it starts adds extra information. Leave the comma out if the clause is needed to show which person, thing, place or time you mean.

Grammar guides call the extra kind a nonrestrictive (or non-defining) clause and the needed kind a restrictive (or defining) clause. Merriam-Webster sums up the punctuation: "There should always be a comma before a nonrestrictive clause, but not before a restrictive clause." The comma changes what the sentence says:

• My brother, who lives in London, is visiting next month. You have one brother, and he happens to live in London.
• My brother who lives in London is visiting next month. You have more than one brother, and you mean the one in London.

Two more rules cover most of what is left. An extra clause in the middle of a sentence needs a comma at each end. And if a when or where clause opens the sentence, put a comma after it. Each word has its own section below, with correct and incorrect examples, followed by which vs that, a checklist and a short quiz.

Comma before which

Use a comma before which when the clause adds a detail about something the reader can already pick out. If the clause sits in the middle of the sentence, close it with a second comma.

• Correct: My first car, which I bought in 2015, still runs.
• Incorrect: My first car which I bought in 2015 still runs.

You only have one first car, so "which I bought in 2015" does not tell the reader which car you mean. It is extra, so it gets a comma at each end.

Leave the commas out when the clause is what identifies the thing:

• Correct: Any car that is parked in the fire lane will be towed. (Many British writers would use which here, also without commas.)
• Incorrect: Any car, which is parked in the fire lane, will be towed.

With commas, the sentence claims that every car is parked in the fire lane. The clause has to stay attached to "any car" to limit which cars you mean.

One use of which always takes a comma: when it comments on the whole idea before it rather than on a single noun. Britannica Dictionary's editors put it this way: clauses "that begin with which and refer to the entire main clause are nonrestrictive."

• Correct: The train was late, which meant we missed the start of the film.
• Incorrect: The train was late which meant we missed the start of the film.

Comma before who

The same test works for people. Use commas when the who clause adds a detail about someone the reader can already identify, often by name. Leave them out when the clause tells the reader which people you mean.

• Correct (extra): Maria, who sits next to me at work, has a new job.
• Correct (needed): Students who finish early can go out for an extra recess.
• Incorrect: Students, who finish early, can go out for an extra recess.
• Incorrect: Maria, who sits next to me at work has a new job.

With commas, the students sentence says that all the students finish early, which is not what you mean. The second Maria sentence opens a pair of commas and never closes it. An extra clause in the middle of a sentence needs a comma at each end.

Comma before where

When where follows a place, it works like which (it means "in which" or "at which"). Use a comma when the place is already clear and the clause just adds a detail. Leave it out when the clause tells the reader which place you mean.

• Correct (extra): We spent a week in Lisbon, where my cousin lives.
• Correct (needed): This is the restaurant where we first met.
• Incorrect: Lisbon where my cousin lives is our next stop. Write Lisbon, where my cousin lives, is our next stop.
• Incorrect: Every room, where food is served, must have a fire exit. Write Every room where food is served must have a fire exit.

The commas in the last example would mean that food is served in every room. Without them, the rule covers only the rooms where food is served.

Where can also open a sentence, meaning "in the place that" or "in situations that." Then the comma goes after its clause, as in Cambridge's example: Where you have to pay a deposit, be sure to get a receipt.

Comma before when

You may have been told to put a comma before when if it joins two complete thoughts. That is not how the rule works. A when clause is not a complete thought: Cambridge describes it as a subordinate clause that needs a main clause to complete its meaning. Joining two complete thoughts is a job for a comma plus and, but or so, or for a semicolon, which is also why however between two sentences needs one (see starting a sentence with "however").

What decides the comma is where the clause sits and what it does:

• The when clause comes first: put a comma after it. When the power went out, I was watching TV.
• The when clause comes after the main clause: usually no comma. I was watching TV when the power went out. Purdue OWL's rule is not to put a comma after the main clause when a dependent clause follows it, except for cases of extreme contrast. Chicago says such a clause takes no comma if it is "essential to fully understanding the meaning of the main clause."
• The clause adds a detail about a time you have already named: use a comma. We moved to Leeds in 2019, when my daughter started school. Chicago's rule is that a dependent clause that is "merely supplementary or parenthetical" takes a comma.

A comma is wrong when the clause is needed, whether it follows a whole clause or a time word such as day, year or moment. In the second pair below, the clause tells the reader which day you mean:

• Correct: I always get hungry when I smell fresh cookies baking.
• Incorrect: I always get hungry, when I smell fresh cookies baking.
• Correct: I still remember the day when we met.
• Incorrect: I still remember the day, when we met.

Which vs that

For extra information about a thing, the guides agree: use which after a comma, never that. Chicago's editors put it plainly: "In contemporary English, the word that never introduces a nonrestrictive relative clause." Cambridge gives the same warning.

For needed information, practice differs:

• American guides prefer that. Purdue OWL says "For a restrictive clause, use that," and Chicago notes that "in American English, which is normally reserved for nonrestrictive clauses."
• British English uses both. Cambridge and the British Council list which or that for things in defining clauses, and the British Council adds that that is "more common and a bit more informal." Chicago also says many writers, "notably in British English," use which this way.
• Merriam-Webster accepts both. Its usage guide says "That and which are both used to introduce a restrictive clause."

So The book which I borrowed is overdue is not a grammar mistake. If you write American English, or for an editor who follows Chicago or Purdue OWL, that is the safer choice. Whichever word you pick, the comma rule stays the same: no comma before a needed clause.

Practice: comma or no comma?

Decide whether each sentence needs commas. The answers are below.

1. Paris where we spent our honeymoon is my favorite city.
2. Anyone who wants a ticket should email me by Friday.
3. I was reading a book when someone knocked on the door.
4. The meeting ran late which meant I missed my bus.
5. My sister who works as a nurse loves her new job.

Answers

1. Add a comma before and after the clause: Paris, where we spent our honeymoon, is my favorite city. Paris is already named, so the clause is extra.
2. No commas. The clause tells the reader who should email you.
3. No comma. The when clause follows the main clause and completes its meaning.
4. Add a comma before which: The meeting ran late, which meant I missed my bus. Here which comments on the whole first clause.
5. It depends. If you have one sister, add commas: My sister, who works as a nurse, loves her new job. If you have more than one sister and mean the nurse, leave them out.

Sources

The rules on this page follow these guides, all checked on October 26, 2026:

• Purdue OWL, Extended Rules for Using Commas: commas around nonessential elements, a comma after an opening when clause, and no comma after a main clause when a dependent clause follows it.
• Purdue OWL, That vs. Which: that for restrictive clauses, which with commas for non-restrictive ones.
• Chicago Manual of Style Q&A on a comma before a dependent clause, quoting CMOS 6.27.
• CMOS Shop Talk, Commas with Restrictive and Nonrestrictive Relative Clauses: the removal test, and which vs that in American and British English.
• Merriam-Webster, which (its That vs. Which usage guide) and Merriam-Webster, restrictive.
• Cambridge Grammar, Relative clauses: defining and non-defining, and its entries for when and where.
• British Council LearnEnglish, Relative clauses: defining relative clauses.
• Britannica Dictionary, Restrictive and Nonrestrictive Clauses.

Related: see the full Grammar & Word Choice guide for more confused-word and punctuation explainers.