A restrictive appositive provides information essential to identifying the phrase in apposition. A non-restrictive appositive provides information not critical to identifying the phrase in apposition. Other phrases for the usage include "pseudo title", "coined title" and "preposed appositive". Conversely, non-restrictive relative clauses are called supplementary, appositive, non-defining or non-identifying relative clauses. A relative clause is not always an appositive. An appositive colon also separates the subtitle of a work from its principal title.