The most common French prepositions are à (to, at, in) and de (of, from, about). When these two prepositions are followed by the definite articles le and les, a contraction needs to be formed. (Note: There’s no contraction with à or de plus la or l’: à la, à l’, de la, de l’.)

Article à + (le/les) de + (le/les)
Le au du
Les aux des

À and de also contract with the different forms of lequel (which one):

Form of Lequel à + (lequel) de + (lequel)
Lequel auquel duquel
Lesquels auxquels desquels
Lesquelles auxquelles desquelles

There’s no contraction with laquelle: à laquelle, de laquelle.