1) Do not place a comma before "that"; 2) Do not place a comma before who or which if the information provided by the relative clause is essential; 3) Place a comma before who or which if the relative clause is not necessary to know who or what the speaker means.
The comma rules regarding the relative pronouns who, which and that in English are as follows:
- Do not place a comma before that:
The man that lives next door is tall.
The man, that lives next door, is tall.
Do not place a comma before who or which if the information provided by the relative clause is essential to understand which person or thing (or what kind of person or thing) the writer is talking about.
The man who lives next door is tall.
The man, who lives next door, is tall.
We need the relative clause "who lives next door" to know exactly who is the man the speaker is talking about. We do not add commas in a situation like this.
Place a comma before who or which only if the relative clause is not necessary to know who or what the speaker means. In this case, the relative clause provides additional information about the person or thing (or the type of person or thing), but this extra information is not essential.
My neighbor Peter, who lives next door, is very tall.
Before saying "who lives next door", the reader knows who you are talking about (your neighbor, Peter). The clause "who lives next door" just adds extra information.
My neighbor Peter who lives next door is very tall.
1. Using Commas with Relative Pronouns
Look at this example:
People who live in cities are more stressed.
"Who live in cities" is a relative clause (a part of a sentence that tell us what type of person or thing the speaker means). Relative clauses begin with relative pronouns, such as who, which, and that.
The children who read at home regularly make the most progress.
"Who read at home regularly" is essential to understand the meaning of the sentence. If we omit this clause, the sentence makes no sense in this context.
The children make the most progress.
The sentence above is incorrect because we are trying to say that only some students, those who read at home regularly, make the most progress. In other words, leaving out commas helps you clarify who or what you are talking about.
My brother, who works as a dentist, is wealthy.
If we take out the relative clause ("who works as a dentist"), the sentence is complete and makes sense. It also provides the most meaningful information (he is wealthy).
2. Using a Comma Before 'Who' or 'Which' May Change the Meaning of a Sentence
Examine this example:
My sister, who is very tall, can speak English and French.
This sentence is correct because you are assuming that the reader knows who you are talking about (your sister). However, if you have two sisters, the reader may need to know which sister you are mentioning. Remember, we must not use commas if the information is essential to know who or what you are talking about:
My sister who is very tall can speak English and French.
I'm planning to go to Spain which is very sunny next month.
In a situation like this, the use of commas is not optional (they are required). The clause before the relative pronoun tells us where you are going (Spain). The relative clause "which is very sunny" just adds extra or additional information about Spain.
I'm planning to go to Spain, which is very sunny, next month.
3. 'Who' and 'Which' vs. 'That' Regarding Comma Usage
Remember that we do not use commas before that.
Paris, that is the most romantic city in the world, is a beautiful place to live.
Rebecca, that lives in New York, is very smart.
In a situation like this we must use who (for persons) or which (for things).
Paris, which is the most romantic city in the world, is a beautiful place to live.
Rebecca, who lives in New York, is very smart.
4. When to Use 'That', 'Who' and 'Which'
- We use who for people (sometimes for animals too).
I don't like movies who have scenes of violence.
I don't like movies that have scenes of violence.
- We use which for things.
My uncle, which is an engineer, went to London last week.
I only buy watches which are waterproof.
- We use that for people or things.
The girl that came last week is smart.
The dish that you prepared yesterday was delicious.
- The use of that is more common than which, but sometimes you need to use which (see next point).
Rebecca works for a company that makes drones.
- If the sentence requires commas, you must use which for things and who for people (not that).
Madrid, which is the capital of Spain, was founded around the year 860 A.C.
Alex, who is very tall, is a good chess player.
- In many situations, you can omit pronouns in relative clauses. You can learn more about this topic by reading our post on how to omit correctly that, who, or which.
5. More Examples with 'Who'
- The waiter who served us was rude.
- What was the name of the person who came yesterday?
- The man who was carrying a suitcase was very polite.
- My friend Olivia, who lives next door, is an engineer.
- What was the name of the person who wanted to talk to you?
- The police caught the girl who stole the wallet.
- Marie Curie, who was the first woman to win a Nobel Prize, discovered the existence of the elements polonium and radium.
- The movie was about a boy who was raised in Paris.
- A firefighter is a person who fights fires and rescues people.
- Do you know who took those pictures?
6. More Examples with 'That'
- The restaurant that was destroyed in the fire has now been rebuilt.
- The protester that was arrested has hired a new lawyer.
- What happened to the hamburger that was on the table?
- A paradox is a statement or proposition that is seemingly contradictory.
- Andorra is a microstate that borders France and Spain.
- Thailand is the only Asian country that was never colonized by a European country.
- The company that was fined for air pollution is now investing in renewable energy.
7. More Examples with 'Which'
- Artificial intelligence, which combines computer science and robust datasets, enables the execution of complex tasks.
- The new stadium, which can hold 20,000 people, has an oval shape.
- We often go to Chicago, which is not far away.
- Inflation, which is pretty high right now, is having an impact on US consumers.
- Yoga, which improves core strength and flexibility, is very popular in India.