Relative adverbs

The relative adverbs where, when & why can be used to join sentences or clauses. They replace the more formal structure of preposition + which used to introduce a relative clause.

Formal structure, preposition + which More common stucture using a relative adverb
That's the restaurant in which we met for the first time. That's the restaurant where we met for the first time.
That picture was taken in the park at which I used to play. That picture was taken in the park where I used to play.
I remember the day on which we first met. I remember the day when we first met.
There was a very hot summer the year in which he was born. There was a very hot summer the year when he was born.
Tell me the reason for which you came home late. Tell me (the reason) why you came home late.
Do you want to know the reason for which he is angry with Sally? Do you want to know (the reason) why he is angry with Sally?