A kind of clause (a group of words built around a subject and verb ) that is a variety of adjectival clause . Relative clauses are used to give extra detail about the subject or object noun of a main clause in a sentence. e.g. A main clause might be, “The butcher sold me some sausages.' and a relative clause could be, ' who works in Tesco's' . The sentence could then become, 'The butcher, who works in Tesco's, sold me some sausages.'
A relative clause usually begins with a relative pronoun such as: that, which, who, whom, although 'that' is often elided as in: 'He knew [that] we were going early.'.