Non-Defining Relative Clauses

A non-defining relative clause adds extra information about a noun that is already identified. The noun doesn't need this clause to be understood — remove it, and the sentence still works.

We already know which Sarah. The clause who lives in London is bonus detail, not identification. Compare this with a defining clause, where the clause is essential: The woman who lives next door is a teacher — without who lives next door, we don't know which woman.

Commas are required

A non-defining clause is always separated from the rest of the sentence by commas. This is the most visible difference from defining clauses, which use no commas at all.

If the clause comes at the end of the sentence, one comma before it is enough — the full stop closes it.

The commas work like parentheses. If you mentally replace them with brackets, the sentence still reads correctly: My brother (who is 30) lives in Berlin. This is a useful test — if the information feels like a side note you could put in brackets, it belongs in a non-defining clause with commas.

Relative pronouns

Non-defining clauses use who, whom, which, and whose. The choice depends on what the pronoun refers to and its role in the clause.

Who and whom

Who is the default for people. Use whom when the pronoun is the object, especially in formal writing.

In everyday English, who often replaces whom even in object position: Daniel, who I met at a conference... Both are acceptable, but whom is more precise.

Which

Which is used for things, animals, and abstract ideas.

Whose

Whose marks possession and works for both people and things.

"That" is not allowed

In defining clauses, that freely replaces who or which. In non-defining clauses, that cannot be used.

Wrong: "My car, that is red, needs a new battery."

This rule has no exceptions. If you see commas around a relative clause, that does not belong there.

The pronoun cannot be dropped

In defining clauses, the relative pronoun can be omitted when it acts as the object: The book I bought is fine. In non-defining clauses, the pronoun must stay, even when it is the object.

Wrong: "Daniel, I met at a conference, now runs his own company."

Wrong: "The hotel, we booked online, turned out to be excellent."

Without the pronoun, the sentence breaks — two independent clauses collide with just a comma between them.

"Which" referring to a whole clause

Which can refer not just to a single noun but to the entire fact described in the previous clause. In this use, which always comes after a comma and comments on the whole situation.

Here which does not refer to exam or attempt — it refers to the fact that she passed. A few more examples:

This pattern is extremely common in both spoken and written English. It often expresses a reaction or consequence.

Quick contrast with defining clauses

The differences between the two clause types are few but strict.

The comma alone changes the meaning. In speech, the same distinction is made with a slight pause before and after the clause.

Common mistakes

Missing commas

Using "that" instead of "who" or "which"

Dropping the relative pronoun

Confusing defining and non-defining