Imagine your friend is trying to decide on a shirt to wear to a party and asks for your opinion. In French, there are two main forms that question could take:
Quelle chemise préfères-tu?
Which shirt do you prefer?
Laquelle de ces chemises préfères-tu?
Which of these shirts do you prefer?
There's a slight but important difference between these two questions. Though quelle and laquelle both mean "which," laquelle more specifically means "which one." Since laquelle is a pronoun, you can simplify the second sentence and just say, Laquelle préfères-tu? (Which one do you prefer?) However, you can't simplify the first one (Quelle préfères-tu?) because quelle is an adjective and therefore always precedes a noun.
Note that quelle and laquelle agree in number (singular) and gender (feminine) with the noun they refer to, chemise. Their other forms are quel/lequel (masculine singular), quels/lesquels (masculine plural), and quelles/lesquelles (feminine plural). As you can see, the pronoun is formed by combining the definite article le, la, or les with the corresponding form of quel.
Besides introducing a question, lequel/laquelle/lesquels/lesquelles can also be used after a preposition. Here they are in action with the prepositions sur (on) and dans (in):
Le territoire sur lequel ils sont installés…
The territory on which they have settled…
Cap. 40, Actus de Quartier: Fête de quartier Python-Duvernois - Part 4
Par exemple, j'ai ma deuxième robe, dans laquelle je chante mon duo.
For example, I have my second dress, in which I sing my duet.
Cap. 25, Melissa Mars: Mozart, L'opéra rock - Part 1
Watch out for the prepositions à (to) and de (of, from) in this construction. Just as à + le becomes au instead of à le, and de + le becomes du instead of de le, à + lequel and de + lequel become auquel (to which) and duquel (from which, of which, about which). In all forms except the feminine singular (à laquelle, de laquelle), à and de combine with the pronoun to form one word:
Masculine singular: duquel (de + lequel), auquel (à + lequel)
Masculine plural: desquels (de + lesquels), auxquels (à + lesquels)
Feminine plural: desquelles (de + lesquelles), auxquelles (à + lesquelles)
An important note about duquel/de laquelle/desquels/desquelles: these constructions are often replaced by the word dont, the subject of our previous lesson. So instead of a sentence like:
Voici le livre duquel je t'ai parlé hier.
Here is the book about which I spoke to you yesterday.
You would more often hear:
Voici le livre dont je t'ai parlé hier.
Here is the book I spoke to you about yesterday.
However, you have to use duquel, de laquelle, etc., whenever the de is part of a prepositional phrase such as près de (near), à côté de (next to), or loin de (far from):
Il est bordé des quais de Valmy et de Jemmapes au bord duquel se trouve le fameux Hôtel du Nord.
It is bordered by the Quais de Valmy and Jemmapes, along which is found the famous Hôtel du Nord.
Cap. 32-33, De nouvelles découvertes avec Marion: Le canal Saint-Martin
Another important note: Though it's common in English to end a clause with a preposition like "about" or "from," you can never do this with de, duquel/de laquelle, etc., or dont. For example, you can say "the book I spoke to you about," but you can never say le livre je t'ai parlé duquel or le livre je t'ai parlé dont. You can only say le livre duquel je t'ai parlé or le livre dont je t'ai parlé (the book about which I spoke to you).
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