A relative clause is a clause that begins with a relative pronoun such as who, whom, whose, which , or that , or a relative adverb such as when, where , or why . A relative clause can be essential or nonessential. A relative clause is essential when you need the information it provides. A relative clause is nonessential when you do not need the information it provides. An essential relative clause does not require a comma to separate it from the rest of the sentence. A nonessential relative clause does require a comma to separate it from the rest of the sentence.