When do I use "Who's" and when do I use "Whose"?

Apostrophe = contraction ("who is"/"who has"). No apostrophe = possessive ("belonging to whom").

What is the most common mistake with "Who's" and "Whose"?

Using "who's" when you need the possessive: "Who's book is this?" (correct: Whose book). Or "whose" for the contraction: "Whose coming tonight?" (correct: Who's coming).

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Which one to use?Intermediatecontractionpronoun

Who'svsWhose

'Who\'s' contracts 'who is' or 'who has'; 'whose' asks or states who something belongs to.

Side-by-side comparison

Who's

contraction

"Who's" is a contraction of "who is" or "who has". Use the apostrophe form when you can replace it with "who is" or "who has". "Who's there?" = who is there?

Who's responsible for the marketing campaign?
I don't know who's coming to the meeting.
Who's already submitted their report?

Whose

pronoun

"Whose" asks or states who something belongs to. "Whose bag is this?" = which person does this bag belong to? "The student whose essay won" = the student to whom the essay belongs.

Whose bag is this on my desk?
The employee whose idea won the award received a bonus.
Could you tell me whose turn it is to present?

Common mistake

•Using "who's" when you need the possessive: "Who's book is this?" (correct: Whose book).
•Or "whose" for the contraction: "Whose coming tonight?" (correct: Who's coming).

Trick to remember it

Test: expand to "who is", if it makes sense, use "who's"; if not, use "whose." "Who is book is this?" = nonsense = "whose"; "Who is coming?" = makes sense = "who's."

Quick rule

•Apostrophe = contraction ("who is"/"who has").
•No apostrophe = possessive ("belonging to whom").

Did you know?

The WHOSE/WHO'S distinction is part of a repeated pattern in English: IT'S/ITS (it's = it is; its = its own), THEY'RE/THEIR/THERE, YOU'RE/YOUR. All share the same principle: the form with apostrophe is the contraction, the form without apostrophe is the possessive. Interestingly, WHOSE can refer to both people and inanimate things in modern English: "the house whose windows are broken" is perfectly correct, though some formal writers still prefer the longer alternative "the house the windows of which are broken." The WHOSE form is now accepted and preferred by most modern style guides.