A relative clause is a part of a sentence that provides additional information about a noun. It usually begins with a relative pronoun such as who, whom, whose, which, or that. For example, in the sentence "The book that I borrowed was fascinating," the clause "that I borrowed" gives more details about the noun book. Relative clauses can be restrictive or non-restrictive. A restrictive clause is essential to the meaning of the sentence, while a non-restrictive clause adds extra information but can be removed without changing the sentence's main idea. For instance, "My sister, who lives in New York, is visiting" includes a non-restrictive clause.