There is a rich variety of elementary particles. There is not also the electron, but also its heavier brothers the muon and the tau particle. The up and downquark that make up protons and neutrons are also not alone: there are strange, charm, bottom and top quarks, that are like them, but just a bit heavier. Together, the particles can be grouped in three generations. In the Standard Model of particle physics, this is considered an experimental fact, but not explained with a deeper physical reason. Theories where the Standard Model is extended with a flavour symmetry can not only explain the fact that elementary particles come in generations, but also shine light on the particular structures seen in the masses of the particles of the different generations.