11.3.1 Formation of lequel, laquelle, lesquels, lesquelles
Probably you have noticed that the relative pronoun (other than qui and que) agree in gender and number with the object that is referred to.
La fille avec laquelle j' ai parlé est malade. (female / singular)
The girl with whom I have talked, is sick.
Le garçon avec lequel j' ai parlé est malade. (male / singular)
The boy with whom I have talked, is sick.
Les filles avec lesquelles j' ai parlé sont malades. (female / plural)
The girls with whom I have talked, are sick.
Les garçons avec lesquels j' ai parlé sont malades. (männlich / plural)
The boys with whom I have talked, are sick.
Probably you have noticed that the relative pronouns lequel, laquelle, lesquels, lesquelles are a combination of an article and quelle. The fact that there is an article leads also to the change in the relative pronouns, when used with certain prepositions (as we have seen before in chapter 5.5.1 and chapter 5.5.2).
L' homme auquel j' ai donné l' argent a disparu.
The man, to whom I have given money, has disappeared.
L' homme duquel nous avons parlé est médecin.
The man about which we have talke, is a doctor.
You see there is a continuous logic within this system.