Prepositions in relative clauses

The relative pronoun can also refer to the object of a preposition. In informal styles, the preposition comes after the verb:

The house which Mozart was born in is now a museum. (defining relative clause)
The house that Mozart was born in is now a museum. (defining relative clause)
The house Mozart was born in is now a museum. (defining relative clause)

Mr Pierce, who I talked to just now, sends you his regards. (non-defining relative clause)

In formal styles, the preposition is placed before the pronouns whom or which:

The house in which Mozart was born is now a museum. (defining relative clause)

Mr Pierce, to whom I talked just now, sends you his regards. (non-defining relative clause)