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?|