Which word signals a nonrestrictive clause?

Nonrestrictive clauses, like all relative clauses (or adjective clauses), begin with a relative pronoun, typically “which,” “who,” “whom,” or “whose” (e.g., “I want to vacation in Majorca, which is part of Spain”).

The relative pronoun “that” is only used with restrictive clauses, never with nonrestrictive clauses.

Nonrestrictive clauses are set off from the rest of the sentence by commas.