The cerebellum, a posterior part of the brain crucial for motor coordination, is composed of folia – functionally distinct units that each receive specific combinations of inputs from the rest of the nervous system. Generation of folia, separated by fissures with anchoring centres at their base, requires the proliferation of granule cell progenitors (gcps) and their differentiation into granule cells (gcs). The basic pattern of folia (relative size, number) is conserved across species. To investigate how gcp behaviour regulates folium geometry, Alexandra Joyner and co-workers (p. 1661) performed a clonal analysis and evaluated the geometry of gc clones, their cell number and cell dispersion with respect to folium size and timing of generation. Gcp division and dispersion were preferentially oriented along the anterior-posterior (AP) axis, which correlates with the preferential AP growth of the cerebellum. Furthermore, gcps do not cross anchoring centres and show different behaviour (geometry and cell number) if they reside in short folia versus long folia. Lastly, by analysing clone geometry in En1+/−;En2−/− mice with shorter folia, the authors show that gcp behaviour does underlie folia size. This study uncovers how the behaviour of neuronal progenitors shapes cerebellar foliation, providing crucial insight into the possible mechanisms that lead to the emergence of a folded neocortex in gyrencephalic mammals.