A relative clause is a part of a sentence that describes a noun. It starts with a word like who, which, or that and contains its own subject and verb. A defining relative clause gives essential information — it tells the reader which person or thing is meant. Without it, the sentence loses its point.
Remove who lives next door, and you get The woman is a doctor — but which woman? The clause defines her.
Defining relative clauses are never separated from the rest of the sentence by commas. This is the key visual difference from non-defining clauses, which are covered separately.
Relative pronouns
Defining relative clauses begin with one of five relative pronouns.
| PRONOUN | USED FOR | ROLE |
|---|---|---|
| who | people | subject or object |
| whom | people | object (formal) |
| which | things | subject or object |
| that | people or things | subject or object |
| whose | people or things | possession |
That is the most flexible pronoun in defining clauses. It can replace both who and which, and it is the most common choice in everyday English.
Whom appears mainly in formal writing. In conversation, most speakers use who or that instead, or drop the pronoun entirely.
Subject relative clauses
When the relative pronoun is the subject of the clause — the one performing the action — use who, which, or that.
In each case the pronoun does the action inside the clause: the student passed, the flight departs, the dog had been living.
You cannot remove the pronoun from a subject relative clause. The student passed the exam received a certificate is not a valid sentence.
Object relative clauses
When the relative pronoun is the object of the clause — receiving the action — you can use who, whom, which, or that.
Here a different word is the subject inside the clause (James recommended, we interviewed, my sister had bought), and the pronoun stands for the object.
Omitting the relative pronoun
In object relative clauses, the pronoun can be dropped entirely. This is extremely common in spoken English and informal writing.
A simple test: if the pronoun is followed by another subject (James, we, my sister), it can be dropped. If the pronoun is followed by a verb (passed, departs, lives), it cannot.
Dropping who here would create The student passed the exam received a certificate — broken English.
That as a universal pronoun
In defining clauses, that works for both people and things and is often preferred over who or which in everyday language.
After something, anything, everything, nothing, all, and superlatives, that is the natural choice.
In formal writing about a specific individual, some readers prefer who over that: The professor who supervised my thesis rather than The professor that supervised my thesis. Both are grammatically correct.
Whose for possession
Whose shows that something belongs to the noun being described. It works for both people and things.
Whose always stays in the clause — it cannot be dropped.
Prepositions in relative clauses
When the verb inside the clause needs a preposition, two positions are possible.
End of the clause (natural in everyday English):
Before the pronoun (formal, mainly in writing):
When the preposition moves to the front, that cannot follow it — use which or whom instead. The hotel at that we stayed is not possible.
No commas
Defining relative clauses sit directly against the noun they describe, with no commas on either side.
Adding commas would change the meaning entirely. The passengers, who arrived late, missed the flight implies all the passengers arrived late — a different sentence with a different fact.
This is the single most reliable way to tell defining and non-defining clauses apart in writing: no commas means defining.
Common mistakes
Dropping a subject pronoun
Adding commas to a defining clause
Using that after a preposition
Doubling the pronoun with a personal pronoun
Using whose incorrectly