Relative clauses provide extra information about nouns by functioning like adjectives. They can be either defining clauses that identify the noun or non-defining clauses that provide unnecessary but interesting information. Relative clauses are introduced by relative pronouns like who, which, that, whose or no pronoun. Defining clauses use a definite relative pronoun and are essential to the meaning, while non-defining clauses use commas and are not essential to the meaning. Relative clauses can refer to people using who/whom or things using which and whose is used to talk about something belonging to a person or thing. When, where and why can also be used in some relative clauses.