who vs whom

who

pron
  • Whoever, he who, they who. 

  • With antecedent as subject. 

  • What person or people; which person or people; asks for the identity of someone; used in a direct or indirect question. 

  • With antecedent as object: whom. 

noun
  • A person under discussion; a question of which person. 

det
  • whose 

whom

pron
  • The person(s) whom; whomever. 

  • As the object of a verb. 

  • As the object of a preposition. 

  • Used to refer to a previously mentioned person or people. 

How often have the words who and whom occurred in a corpus of books? (source: Google Ngram Viewer )