7.4.2 A comparison between relative and interrogative clauses
When the wh-element is part of a PP it has the option of moving alone, a strategy known as preposition stranding, or of taking the whole PP with it, a strategy known as pied-piping (after the story of the Pied Piper of Hamlin, who played his pipes and the rats followed him – the connection between prepositions and rats is, however, mysterious). If that were a relative pronoun, we might expect the same options to be available in that-relatives. But this is not true: