What Is a Relative Clause?
A relative clause is a type of dependent clause that modifies a noun or pronoun, giving extra information about it. It is introduced by a relative pronoun (who, whom, which, that, whose) or a relative adverb (where, when, why). The noun or pronoun being described is called the antecedent.
Think of a relative clause as a description that is glued directly onto the noun it describes. Without it, your sentence still makes sense — but with it, your meaning becomes sharper and more specific.
The woman who lives next door is a doctor. I returned the book that I had borrowed. This is the city where I grew up. He spoke to the manager, who was very helpful.
In each sentence above, the relative clause (shown in the examples after the comma or after the noun) tells us which woman, which book, or which city we mean — or it adds a useful detail about a person or place already identified.
Relative clauses always come immediately after the noun they describe. Placing them elsewhere in the sentence creates confusion. Compare these two sentences:
Correct: The car that I bought broke down after a week. Confusing: The car broke down that I bought after a week.
If you can ask "Which one?" or "What kind?" about the noun, a relative clause is probably the most natural way to answer that question in English.
Defining vs Non-Defining Clauses
This is the most important distinction in the whole topic. Every relative clause is either defining (also called restrictive) or non-defining (also called non-restrictive). Understanding the difference will change how you use commas — and how clearly you communicate.
Defining relative clauses identify which person or thing you mean. They are essential to the meaning of the sentence. Remove them and the sentence either loses its meaning or becomes too vague.
The student who studies hardest usually gets the best grades. I want to visit the restaurant that you recommended. She wore the dress which she had bought in Paris.
In the first example, the clause who studies hardest tells us which student — not just any student. Without it, the sentence says very little that is useful.
Non-defining relative clauses, by contrast, add extra information about a noun that is already fully identified. The information is interesting but not essential. Remove the clause and the main meaning survives intact.
My sister, who lives in Madrid, is a graphic designer. The Eiffel Tower, which was completed in 1889, attracts millions of visitors. We met Professor Sato, whose research focuses on climate science.
Notice that My sister already identifies a specific person — there is only one. The clause who lives in Madrid simply adds a detail. These clauses are always separated by commas (see the Commas section for more detail).
A quick test: try removing the relative clause. If the sentence still makes complete sense AND still refers to the same specific person or thing, it is non-defining. If removing it makes the sentence vague or meaningless, it is defining.
Choosing the Right Pronoun
Choosing the correct relative pronoun is straightforward once you know two things: what the antecedent is (a person, a thing, or a place) and what role the pronoun plays in the clause (subject, object, or possessive).
Who — used for people when the relative pronoun is the subject or object of the relative clause.
The teacher who inspired me most retired last year. (subject) The colleague who I respect most is leaving the company. (object)
Whom — the formal object form for people. In everyday spoken English, who often replaces it, but whom is required after a preposition and in formal writing.
The director whom she admired gave a speech. (formal object) The person to whom I spoke was extremely helpful. (after preposition) This is the candidate for whom we voted. (after preposition)
Which — used for things and animals. In non-defining clauses, only which (not that) is used.
The report which landed on my desk was fifty pages long. She owns a cat which follows her everywhere. The bridge, which was built in the 1930s, is still in excellent condition.
That — used for both people and things in defining clauses only. It cannot be used in non-defining clauses.
The bus that goes to the airport leaves every hour. The scientist that discovered the compound won an award. Everything that I own fits into two suitcases.
Whose — the possessive form, used for people and things to show belonging.
The student whose essay won the prize was very surprised. We live in a house whose roof needs replacing. I met a writer whose novels have been translated into twenty languages.
Where — a relative adverb used for places.
This is the town where my grandparents were born. She found a café where she could work quietly. They returned to the beach where they had first met.
Omitting the Relative Pronoun
One of the most useful — and most misunderstood — features of English relative clauses is that you can sometimes leave out the relative pronoun entirely. This is called a contact clause, and it sounds very natural in everyday English.
You can omit the relative pronoun only when it is the object of the defining relative clause — that is, when the clause has its own subject that is different from the relative pronoun.
The film (that) I watched last night was incredible. The woman (who/whom) you met at the party is my cousin. Is this the book (which/that) you were looking for? The hotel (that) we stayed in had a rooftop pool.
In all these examples, the relative clause has its own subject (I, you, we), so the pronoun is not needed to carry any grammatical weight. You can drop it or keep it — both are correct.
However, you cannot omit the pronoun when it is the subject of the relative clause.
Correct: The man who called you left a message. Incorrect: The man called you left a message. Correct: I read a book that changed my thinking. Incorrect: I read a book changed my thinking.
You also cannot omit the pronoun in non-defining clauses, or when it follows a preposition.
Non-defining: My laptop, which I use every day, needs a new battery. After prep: The club to which she belongs meets on Thursdays.
A simple way to check: cover the relative pronoun and ask, "Does the clause now have a subject?" If yes — there is already a subject in the clause — you can safely drop the pronoun. If no, keep it.
Reduced Relative Clauses
A reduced relative clause is a shortened version of a full relative clause. Instead of writing a complete clause with a subject and verb, you use a participle phrase — either a present participle (-ing) or a past participle (-ed / irregular form). This makes your writing more concise and often more elegant.
Reducing active clauses (present participle)
When the verb in the relative clause is active and continuous, replace who/which + be + -ing with just the -ing form.
Full: The man who is standing by the door is the security guard. Reduced: The man standing by the door is the security guard. Full: Students who are taking the exam should arrive early. Reduced: Students taking the exam should arrive early. Full: The train which is arriving on platform three goes to the coast. Reduced: The train arriving on platform three goes to the coast.
Reducing passive clauses (past participle)
When the verb in the relative clause is passive, replace which/who + be + past participle with just the past participle.
Full: The documents which were signed by both parties are legally binding. Reduced: The documents signed by both parties are legally binding. Full: The bridge that was built in the last century is under repair. Reduced: The bridge built in the last century is under repair. Full: Any applicant who is selected will be notified within a week. Reduced: Any applicant selected will be notified within a week.
Reduced relative clauses work best in defining contexts. They are common in formal writing, academic texts, and journalism. In speech, full relative clauses often sound more natural.
Use -ing for active meanings (the thing is doing something) and the past participle for passive meanings (something was done to the thing). Getting this right prevents common errors like writing "the letter writing by her" instead of "the letter written by her."
Commas and Punctuation
Commas are not a matter of style with relative clauses — they carry grammatical meaning. Using or omitting a comma changes the meaning of a sentence, sometimes dramatically.
No commas — defining clauses
Defining relative clauses are written with no commas. The clause is part of the core meaning of the sentence.
The employees who work overtime receive extra pay. (= only some employees — those who work overtime) The package that arrived this morning is on your desk. Children who eat a balanced diet tend to have more energy.
With commas — non-defining clauses
Non-defining relative clauses are enclosed in commas (or a comma at the start if the clause comes at the end of the sentence). Think of the commas like parentheses — they signal that this information is additional, not essential.
My father, who worked as an engineer, always loved mathematics. (= I only have one father; the clause adds information) The new policy, which takes effect next month, will affect all staff. We spoke to Dr. Reeves, whose research we had read about online.
Now see how the comma changes the meaning entirely:
No comma: The students who failed the test had to retake it. (= only the students who failed — not all students) With comma: The students, who failed the test, had to retake it. (= all the students failed the test — surprising information!)
In non-defining clauses, that cannot replace who or which, and the relative pronoun can never be omitted.
Read your sentence aloud and ask: "Am I using the relative clause to tell the reader which one, or am I just adding a bonus fact?" Bonus facts get commas. Identifying information does not.
Common Mistakes
Even advanced learners make the same handful of errors with relative clauses. Here are the most frequent ones — and exactly how to fix them.
Mistake 1: Using that in a non-defining clause
That is only for defining clauses. Non-defining clauses always use who or which.
Wrong: London, that is the capital of England, is a very large city. Right: London, which is the capital of England, is a very large city. Wrong: My brother, that lives in Canada, is visiting next month. Right: My brother, who lives in Canada, is visiting next month.
Mistake 2: Leaving a duplicate pronoun inside the clause
When the relative pronoun refers to the subject or object of the clause, do not repeat it with another pronoun (him, it, her, them).
Wrong: The man who I met him was very kind. Right: The man who I met was very kind. Wrong: The book that I bought it is on the shelf. Right: The book that I bought is on the shelf.
Mistake 3: Confusing whose with who's
Whose is a possessive relative pronoun. Who's is a contraction of who is or who has. They are never interchangeable in writing.
Wrong: The student who's essay was selected received a prize. Right: The student whose essay was selected received a prize. Who's (correct): She is the teacher who's been here the longest.
Mistake 4: Using which to refer to a person
Which refers to things and animals, not people.
Wrong: The doctor which treated me was very thorough. Right: The doctor who treated me was very thorough.
Mistake 5: Missing or misplacing commas
Forgetting commas in non-defining clauses — or adding them to defining clauses — changes the meaning of the sentence. Always ask whether your clause is identifying something or simply commenting on it.
Wrong (comma in defining): The film, that won the award, was very moving. Right: The film that won the award was very moving. Wrong (missing comma): My mother who is a nurse works long hours. (Implies you have more than one mother.) Right: My mother, who is a nurse, works long hours.
Mistake 6: Using -ing in a reduced clause when the meaning is passive
Wrong: The report writing by the committee was excellent. Right: The report written by the committee was excellent. Wrong: The window breaking during the storm has been replaced. Right: The window broken during the storm has been replaced.
When you finish writing a sentence with a relative clause, run through this quick checklist: Is the pronoun correct for the antecedent (person vs thing)? Are commas present or absent for the right reason? Is there a duplicate pronoun hiding inside the clause? A few seconds of checking will catch the most common errors every time.
Frequently Asked Questions
What is the difference between 'who' and 'whom' in relative clauses?
'Who' is used when the relative pronoun is the subject of the clause (the person doing the action): 'The man who called you left a message.' 'Whom' is the object form, used when the pronoun receives the action or follows a preposition: 'The person to whom I spoke was helpful.' In everyday spoken English, 'who' is widely accepted in both positions, but 'whom' is required in formal writing and always after a preposition.
Can I use 'that' instead of 'which' or 'who'?
You can use 'that' instead of 'who' or 'which' in defining (restrictive) relative clauses only. For example: 'The house that she bought needs repairs' or 'The doctor that treated me was kind.' However, 'that' cannot be used in non-defining clauses. You must use 'which' for things and 'who' for people in non-defining clauses: 'My car, which is ten years old, still runs well.'
When can I leave out the relative pronoun?
You can omit the relative pronoun when it is the object of a defining relative clause — that is, when the clause already has its own subject. 'The film (that) I watched was wonderful' — here 'I' is the subject of the clause, so 'that' is optional. You cannot omit the pronoun when it is the subject of the clause ('The woman who called is waiting'), in non-defining clauses, or after a preposition.
How do commas change the meaning of a relative clause?
Commas make a relative clause non-defining, meaning it adds extra information about something already identified. Without commas, the clause is defining — it tells the reader which specific person or thing you mean. For example: 'The students who passed the test celebrated' (only the students who passed) versus 'The students, who passed the test, celebrated' (all the students passed — the clause is just adding that fact).
What is a reduced relative clause?
A reduced relative clause is a shorter form that uses a participle instead of a full relative clause. Active meanings use the present participle (-ing): 'The woman standing at the door' (= who is standing). Passive meanings use the past participle: 'The letter sent yesterday' (= which was sent). Reduced clauses are common in formal and written English, making sentences more concise.
Can 'whose' be used for things as well as people?
Yes. Although 'whose' is most commonly associated with people, it is also correct to use it for things when showing possession or a relationship of belonging: 'We stayed in a hotel whose rooms overlooked the sea' or 'She chose a route whose scenery was breathtaking.' This use is standard in both formal and informal English.