In American English, which is normally used in non-restrictive clauses (offset by commas), and that is normally used in restrictive clauses (not offset by comma

In American English, which is normally used in non-restrictive clauses (offset by commas), and that is normally used in restrictive clauses (not offset by commas). Using which in a restrictive clause like The senator which represents me... or The road which runs from X to Y... is somewhat uncommon. Both of those examples sound better in American English using that in place of which. If the subjects didn't need disambiguation, though, that would no longer be appropriate, e.g., Senator Jones, who represents me, ... or Foobar Street, which runs from X to Y, ... As Cushman points out, though, which used on a person doesn't quite work. That can work on a person, though, but who/whom can work for people in both restrictive and non-restrictive clauses. See http://owl.english.purdue.edu/owl/resource/645/01/ http://owl.english.purdue.edu/owl/resource/645/01/ if you want to read a well-structured and thorough explanation.