The interrogative pronouns are "who", "what", and "which" (all of them can take the suffix "-ever" for emphasis). The pronoun "who" refers to a person or people; it has an oblique form "whom" (though in informal contexts this is usually replaced by "who"), and a possessive form (pronoun or determiner) "whose". The pronoun "what" refers to things or abstracts. The word "which" is used to ask about alternatives from what is seen as a closed set: "which (of the books) do you like best?" (It can also be an interrogative determiner: "which book?"; this can form the alternative pronominal expressions "which one" and "which ones".) "Which", "who", and "what" can be either singular or plural, although "who" and "what" often take a singular verb regardless of any supposed number. For more information see "who".