Pick out the relative clause in the following sentence:

The cat, which scratched my mum, is wild. 

A relative clause is a type of subordinate clause that starts with a relative pronoun (such as which, who, whom, whose, or that) and functions as an adjective to provide more information about a noun. In this sentence, the clause "which scratched my mum" gives extra detail about "the cat." Because it is non-essential information (the sentence "The cat is wild" still makes sense without it), it is set off by commas.