Definition · Plain-language
Relative clause
A relative clause is a type of dependent clause that modifies a noun or pronoun using a relative pronoun — who, whom, whose, which or that — to give more information about it.
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Defining vs non-defining relative clauses
The most important distinction in relative clauses is between defining and non-defining. A defining (restrictive) relative clause identifies precisely which person or thing is being referred to; remove it and the sentence loses a critical piece of information. "The student who passed the exam was awarded a prize" — without the clause, we do not know which student. No commas are used. A non-defining (non-restrictive) relative clause adds information about a noun that is already clearly identified; it can be removed without changing the core meaning, and it is enclosed in commas. "Ms Chen, who chairs the committee, approved the proposal" — remove the clause and the sentence still makes sense. This distinction matters for style and also affects the choice of pronoun: that is acceptable in defining clauses; which is standard in non-defining clauses.
Choosing the right relative pronoun
The choice of pronoun depends on the antecedent and the clause's grammatical role. Who refers to people acting as subject in the clause: "the author who wrote the paper". Whom refers to people acting as object or following a preposition: "the author whom we cited"; "the editor to whom the paper was submitted". Whose indicates possession for both people and things: "a study whose methodology is sound". Which refers to things and animals (and is used in non-defining clauses): "the experiment, which was repeated three times". That can refer to people or things in defining clauses. In British English, that is particularly common in defining clauses for things; some style guides (APA, Chicago) discourage that for people and prefer who.
Punctuation and common errors
Punctuation is determined entirely by whether the clause is defining or non-defining. Defining clauses take no commas; non-defining clauses are set off by commas (or dashes or parentheses for stronger separation). A common error is using which without commas in what should be a defining clause — many editors change this to that as a matter of style. Another error is using that in a non-defining clause, which is grammatically incorrect in formal writing: "My supervisor, that has retired, …" should be "My supervisor, who has retired, …". A third common error is omitting the relative pronoun in formal writing: "The study she conducted" is acceptable in informal English but should be "The study that she conducted" or "The study which she conducted" in academic prose.
Key facts
At a glance
- Definition: a dependent clause modifying a noun, introduced by a relative pronoun
- Relative pronouns: who (subject, people), whom (object, people), whose (possession), which (things), that (defining clauses)
- Defining/restrictive: identifies which noun — no commas
- Non-defining/non-restrictive: adds extra information — enclosed in commas
- That vs which: that for defining; which for non-defining (with commas)
- That vs who: who preferred for people in formal writing
Common misconceptions
What people often get wrong
Often heard: "Which" and "that" are interchangeable in relative clauses.
Actually: In formal writing they serve different functions. That is used in defining (restrictive) clauses, with no commas. Which is used in non-defining (non-restrictive) clauses, which are enclosed in commas. Using which in a defining clause without commas is common but is considered an error by many style guides.
Often heard: Non-defining relative clauses are optional stylistic additions with no grammatical impact.
Actually: Non-defining clauses affect meaning by providing additional information. Although the sentence is grammatically complete without them, including or omitting them can change what the reader understands about the noun being modified, especially in legal or technical contexts.
Often heard: "That" should never refer to people.
Actually: That can refer to people in defining relative clauses: "the scientist that discovered it". This is grammatically acceptable, though many style guides prefer who for people in formal writing.
Going deeper
