Using duquel, de laquelle etc with prepositional phrases with "de"

A relative clause can be introduced with a prepositional phrase + relative pronoun in English, as follows:

The tree close to which you are sitting is an oak tree.
The tree (which) you are sitting close to is an oak tree.


Now let's look at such cases in French:

L'arbre près duquel tu es assis est un chêne.The tree close to which you are sitting is an oak tree.

Voici les amis au sujet desquels nous sommes inquiets.Here are the friends about whom we are worried.
Here are the friends we are worried about.

Voici la date à partir de laquelle la loi prend effet.Here's the date when [lit: from which] the law comes into effect.

Merci à Léa et Juliesans l'aide desquelles rien n'aurait été possible.Thanks to Léa and Julie, without the help of whom nothing would have been possible.
Thanks to Léa and Julie, without whose help nothing would have been possible.

In French, when introducing a relative clause with a prepositional phrase including de*, you use the relative pronoun duquel, de laquelle, desquels or desquelles to refer to both living beings and inanimate objects.

Note that these pronouns agree in gender and number with the object they refer to.  

ATTENTION:
Unlike in English, you can never place the prepositional phrase+duquel at the end of the clause in French, nor can you separate the phrase from duquel or de laquelle.

The tree (which) you are sitting close to is an oak tree

-> You cannot say any of these:
L'arbre tu es assis près duquel est un chêne.
L'arbre duquel tu es assis près est un chêne.

 

Can we use de qui / dont here?
De qui is also an option, though much less elegant, and only to refer to a singule person:
Le garçon à côté de qui tu es assise a de beaux yeux.The boy (whom) you're sitting next to has beautiful eyes.
Dont or de quoi are completely incorrect in this context:
L'arbre près dont tu es assis...
L'arbre près de quoi tu es assis...
Exceptions
To express the way [which], you will use la manière dont / la façon dont:
J'aime la manière dont tu rougis quand je te fais un compliment.I like the way you blush when I pay you a compliment.

Je ne comprends pas la façon dont tu plies tes vêtements.I don't understand the way you fold your clothes.

 
*Here's a non-exhaustive list of prepositional phrases ending with de:
à la fin de   -  at the end of
au début de  -  at the beginning of
au milieu de  -  in the middle of
avec l'aide de  -  with the help of
à cause de  -  due to 
au sujet de  -  about
au-dessus de  -  above
à partir de  - from
À côté de, en face de, à l'extérieur de, à l'intérieur de, près de, loin de, au coin de (prepositions) 
etc

See also Using de qui, dont, duquel with prepositional verbs with "de" = of/about whom, of/about whichDont = Whose and Dont = Including, [quantity] of which

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Examples and resources

Le garçon à côté de qui tu es assise a de beaux yeux.The boy (whom) you're sitting next to has beautiful eyes.
Le bâtiment en face duquel j'habite est très connu.The building I live opposite to is very famous.
Je ne comprends pas la façon dont tu plies tes vêtements.I don't understand the way you fold your clothes.
Il blâme la neige, à cause de laquelle tout a dû être annulé. He's blaming the snow, because of which everything had to be cancelled.
Le garçon à côté duquel tu es assise a de beaux yeux.The boy next to whom you're sitting has beautiful eyes.
The boy you're sitting next to has beautiful eyes.
J'aime la manière dont tu rougis quand je te fais un compliment.I like the way you blush when I pay you a compliment.
Elle adorait les champs de blé, au milieu desquels elle se perdait avec délice.She loved wheat fields, in the middle of which she would lose herself with delight.
C'est le film à la fin duquel Olivier était en larmes ?Is that the film at the end of which Olivier was in tears?
Voici la date à partir de laquelle la loi prend effet.Here's the date when [lit: from which] the law comes into effect.
Voici la table au-dessus de laquelle je veux suspendre le lustre.Here's the table above which I want to hang the chandelier.
Voici les amis au sujet desquels nous sommes inquiets.Here are the friends about whom we are worried.
Here are the friends we are worried about.
Merci à Léa et Juliesans l'aide desquelles rien n'aurait été possible.Thanks to Léa and Julie, without the help of whom nothing would have been possible.
Thanks to Léa and Julie, without whose help nothing would have been possible.
L'arbre près duquel tu es assis est un chêne.The tree close to which you are sitting is an oak tree.
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