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Who or whom? That or which?

"Relative pronouns are used at the beginning of the subordinate clause which gives some specific information about the main clause." (Source: The OWL at Purdue: Relative Pronouns)

Relative pronouns are that, who, whom, whose, which, where, when, and why.

Who versus whom?

Who and whom are pronouns used only to refer to people. Use "who" when you refer to the subject of a clause and "whom" when you refer to the object of a clause (for information regarding subjects versus objects, please refer to Sentence Elements).

For example:

  • Joe, who likes blue, met Bob, whom he had never met before. ("who likes blue" refers to Joe, who is the subject of the sentence; "whom he had never met before" refers to Bob, who is the object of the sentence; "met" is the verb)
  • To whom are you speaking? ("You" is the subject; "whom" is the object"; "speaking" is the verb)

Avoid ending sentences in a preposition, which is why this sentence isn't correctly written as "Whom are you speaking to?". Please refer to Prepositions for more information.

That versus which?

According to the Publication Manual of the American Psychological Association (2010),

that clauses (called restrictive) are essential to the meaning of the sentence. . . which clauses can merely add further information (nonrestrictive) or can be essential to the meaning (restrictive) of the sentence. APA prefers to reserve which for nonrestrictive clauses and use that in restrictive clauses. (p. 83)

For example:

  • I chose the book that I like the best. ("that I like the best" is essential to the meaning of the sentence because it specifies which book was chosen)
  • My book, which is red, is my new favourite. ("which is red" adds further information to the sentence and is therefore a nonrestrictive clause)

For more information, see the Grammar Girl's "Which or That?".

For more information regarding relative pronouns, please refer to The OWL at Purdue: Relative Pronouns: Introduction and General Usage in Defining Clauses and The OWL at Purdue: Relative Pronouns in Non-Defining Clauses.