The relative pronouns who and which can be used to identify or define nouns. They tell us which person or thing we are talking about.
- The man who had cheated me was arrested by the police.
Here the relative clause introduced by the relative pronoun who identifies the noun man. Which man? – the one who cheated me.
- The book which you see on the table is my favorite. (Which book? – the one you see on the table)
Such a relative clause which defines or identifies its antecedent (the noun phrase) is called a defining or restrictive relative clause.
Restrictive relative clauses follow immediately after the noun that they modify. They are not separated by pauses in speech or commas in writing. Note that a restrictive relative clause cannot be left out without affecting the meaning of the sentence.
When a relative clause merely gives some additional information about the noun, it is called a non-defining or non-restrictive relative clause. Note that a non-restrictive relative clause is not necessary for the identification of the noun.
- My brother, who is a doctor, lives abroad. (Here the relative clause who is a doctor doesn’t identify the noun my brother. It merely gives some additional information.)
- Ann, who works with my brother, is a post graduate in psychology. (Here the relative clause who works with my brother merely gives some additional information about Ann.)
Note that in writing a non-defining or non-restrictive relative clause is separated from the main clause by commas. A non-defining relative clause can be easily left out.
For example, the pair of sentences given below convey the same information. The second sentence just happens to give some additional information.
- Ann is a postgraduate in psychology.
- Ann, who was my senior at university, is a postgraduate in psychology.