We use relative clauses and relative pronouns like who, which, where to introduce them in order to identify people and things or to give more information about them.

• That boy who is standing at the bus stop over there is my little brother.
• My new camera which I bought on the internet last week is broken.
• The High Street jeweller's which bought and sold silver and where you could get a good price by bargaining has closed down.
  

where / in which / at which

In which and at which are sometimes used as more precise sounding alternatives to where to introduce relative clauses after nouns referring to place:

• Near where I live there's a wood where you can find woodpeckers.
  
• Near where I live there's a wood in which you can find woodpeckers.
• The fancy-dress party, where the men all turned up as gangsters, was held in Manhatten.
• The fancy-dress party, at which the men all turned up as gangsters, was held in Manhatten.
  

when / on which

On which is sometimes used as a more precise sounding alternative to when to introduce relative clauses after nouns referring to time:

• The day when I'm forced to give up riding will be a sad day for me.
  
• The day on which I'm forced to give up riding will be a sad day for me.

position of prepositions

Note that in questions the preposition is more frequently placed at the end of the clause. It can also be placed before the relative pronoun where it sounds more formal:

• In which street does he live?
  
• Which street does he live in?
  
  
• He lives in the street where all the houses are surrounded by high fences.
• He lives in the street in which the houses are surrounded by high fences
  
  
• For which organisation does he work?
• Which organisation does he work for?
  
  
• He works for a spy network, about which I know nothing.
• He works for a spy network (which) I know nothing about.
  

Note from examples above and below that putting the preposition at the end of the clause is usually also possible in statements:

• The people with whom he worked have all been arrested. (Formal)
• The people (who) he worked with have all been arrested. (Informal)
  
  
• This is the bedroom in which he was murdered. (Formal)
• This is the bedroom (that) he was murdered in. (Informal)

Note from these examples, that in statements when the preposition is placed at the end of the clause, we can use that instead of who or which or we can omit the relative pronoun completely!

preposition + relative pronoun

A wide range of prepositions are often used in prepositional structures with relative pronouns who and which to introduce relative clauses. In most cases, the prepositions retain their original meaning. Compare the following:

• That post marks the beginning of the mined area, beyond which it is inadvisable to go.
  
  
• In the clearing lay the badly injured soldier, above whom birds of prey were circling.
  
  
• We passed a giant toadstool in the forest, under which fairies were sitting.
  
  
• They had collected the sap from the sugar maple trees, from which maple syrup is manufactured.
  
  
• Before us we could see a forest orchid of which there are many varieties.
  
  
• An Austrian naturalist, with whom I worked closely in the Eighties, discovered this particular orchid.

Note that when the relative pronoun is placed immediately after the preposition we can't use who instead of whom, and we can't use that or zero pronoun either.

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