What is whose?

Whose is a possessive pronoun and a possessive adjective. It indicates ownership or belonging.

• As a possessive pronoun, it replaces a noun to show ownership. For example: "Whose is this pen?" (It implies "Whose pen is this?")
• As a possessive adjective (also known as a possessive determiner), it modifies a noun to show ownership or association. For example: "Whose book is this?" It is used to ask which person the noun belongs to.

Whose should not be confused with who's, which is a contraction of "who is" or "who has".