pronoun that is used to ask an addressee about the identity of a referent whose identity is unknown to the speaker. Example: in Who ate my cookie?, who is an interrogative pronoun; the identity of the cookie eater is unknown to the speaker, who is asking the hearer to provide the referent's identity. The interrogative form may also be a modifier rather than a pronoun: in Which book is required reading?, which is an interrogative modifier denoting the missing information about the book that is required reading. (Sections 3.4.2, 12.3.1)