Relative clauses

Defining and non-defining relative clauses, comma rules, which versus that, and using which to comment on a whole clause.

All grammar rules

Marking criterion: Grammatical Range and Accuracy
Where it pays off: Writing Task 1 · Writing Task 2 · Speaking Part 3

Relative clauses let you add information without starting a new sentence, which is why they appear constantly in band 7 and above writing. The single distinction that matters is whether the clause identifies which thing you mean, or simply adds extra detail, because that decides the commas.

Defining clauses identify; non-defining clauses add

Remove the clause. If the sentence no longer identifies who or what you mean, the clause is defining and takes no commas.

Defining: 'Students who study abroad often adapt faster.' Only those students, no commas.
Non-defining: 'My sister, who studies abroad, adapts quickly.' Extra information about one known person, commas required.
Non-defining clauses cannot use 'that'. 'My sister, that studies abroad' is wrong.
In defining clauses, both 'who' and 'that' work for people, and both 'which' and 'that' work for things.

Choosing the relative pronoun

Five pronouns cover everything you need.

who for people, which for things, that for either in defining clauses only.
whose for possession, for people and things alike: 'a country whose economy depends on tourism'.
where for places, when for times: 'the period when unemployment peaked'.
Use 'in which' rather than 'where' when the noun is abstract: 'a situation in which costs exceed revenue'.

Which can comment on the whole clause

A non-defining 'which' after a comma can refer to the entire preceding idea. This is a natural, high-value structure in Task 1 and Task 2 alike.

'Car ownership doubled over the period, which put considerable pressure on urban infrastructure.'
Only 'which' does this. 'What' cannot: 'which put pressure', never 'what put pressure'.
Keep it to one per paragraph; chained 'which' clauses become hard to follow.

Omission and prepositions

Two refinements that read as controlled rather than showy.

You can omit the pronoun when it is the object of the clause: 'the book (that) I read'. You cannot omit it when it is the subject.
Do not add a second pronoun: 'the book that I read it' is wrong.
In formal writing, the preposition can move forward: 'the extent to which this is true', 'the period during which sales fell'.

Corrected examples

Incorrect: People which live in cities travel less by car.

Correct: People who live in cities travel less by car.

'Which' is for things. Use 'who' or 'that' for people.

Incorrect: The chart shows four countries, that all recorded growth.

Correct: The chart shows four countries, all of which recorded growth.

Non-defining clauses never use 'that'.

Incorrect: Unemployment fell sharply, what improved consumer confidence.

Correct: Unemployment fell sharply, which improved consumer confidence.

'Which' comments on a whole clause; 'what' cannot be used this way.

Quick checklist

No commas around a clause that identifies which one you mean.
Commas, and never 'that', around extra information.
'Whose' works for things as well as people.
Use one comment 'which' clause per paragraph at most.

Apply it to a real response and check the length and structure.

Open the writing checklist