Longman Dictionary of Contemporary English - who

~ pron 1 used in questions to ask what person or people  (Who's that woman over there? | Did they find out who stole the money? | Who did you stay with?) 2 used in a question or statement to show what person or people you are talking about  (Do you know the people who live over the road? | A postman is a man who delivers letters.) 3 used especially in written language after a comma, to add more information about a person or people  (I discussed it with my brother, who is a lawyer.) 4 who are you to...? used to say that someone should not judge someone or something because they have faults themselves or do not have the necessary experience  (Who am I to say how you should bring up your kids; I don't have any.) 5 who's who the people within a particular organization or group and how important each person is, what their job is etc  (I'm just getting to know who's who in the department.)  ( USAGE NOTE: WHO FORMALITY Except in very formal English, who is used instead of whom as an object in all kinds of questions Who did you see? | I wonder who she married in the end. | Who was she dancing with? Compare the much more formal With whom was she dancing? The same is true when who is used in relative clauses that add information but do not restrict the meaning of the noun they follow, often after a comma This is Jed, who you haven't met before/who I work for (=this is Jed, and I work for him). In relative clauses that restrict the meaning of the n, even who is rare as an object in informal or spoken English. Instead, that or nothing is used I'm the person (that) you need to see/speak to (NOT which). It would be very formal to say ...to whom you need to speak. However, whom must be used immediately after a prep, and you can avoid this by rephrasing your sentence. For example, instead of saying To whom are you sending that letter? It is much more natural to say Who are you sending that letter to? You can also use that instead of who when it is the subject of a relative clause I hate people who/that can't stop talking. GRAMMAR You can use who or that when a word for a group of people like family or team is followed by a plural word (N.B. this is only usual in British English) a family who quarrel among themselves. When such words are followed by a singular v, you usually use which or that a team which/that has won most of its games )