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Relative clauses

Relative clauses provide information about something or someone we have already specified. Relative clauses have three elements:

  • They have a subject and a verb
  • They begin with a relative pronoun (which, who, whom, whose, when, where, that)
  • They answer the questions ‘how many’, ‘what kind’ or ‘which one’:


Using ‘that’ is most correct, but in relative clauses ‘that’ and ‘which’ are now used interchangeably. In the example above, it is not all drug related problems that can potentially interfere with desired health outcomes. It is only those problems which/that arise from drug therapy (relative clause). The clause answers the question, ‘what kind of drug-related problems?’

Types of relative clauses

There are two types of relative clauses:

  1. Restrictive relative clauses can begin with who, which or that and provide information about a noun or pronoun that helps readers to understand more about the noun. The clause follows the noun without a comma.sect-02-08
  2. Non-restrictive relative clauses can begin with who or which and add information about a clearly identified noun or pronoun. That is not used for non-restrictive relative clauses.sect-02-09sect-02-09-b

As opposed to the restrictive relative clauses above, the meaning here is that ALL drug related problems arise from drug therapy, which rationally cannot be. Therefore, although the sentence can be punctuated like this, it would most likely be meant and written with a restrictive clause.

The exercise on this page contains examples of relative clauses.


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