What is the difference between the books which were on the table once belonged to my aunt and the books, which were on the table, once belonged to my aunt? In the first sentence the speaker uses the relative clause to pick out a subset of books (the ones on the table) and imply a contrast with some other set of books. In the second sentence the size of the set of books referred to is unaffected by the relative clause; the speaker merely offers the additional information that they happen to be on the table.This distinction is between restrictive and non-restrictive relative clauses. In writing, a non-restrictive relative clause is set off within commas, while in speech the difference is expressed by a difference in intonation. Ignorance of the distinction can lead to unintentionally comic effects: for example, strictly speaking, the relative clause in if you are in need of assistance, please ask any member of staff who will be pleased to help implies contrast with another set of staff who will not be pleased to help. A comma is needed before who.