In this issue of Neuron, Wang et al. and Machold and Fishell present contrasting molecular fate maps of Math1, which redefine the derivatives of the embryonic rhombic lip and offer a conceptual overhaul of cerebellar and precerebellar development. These fate maps identify a common developmental thread linking diverse, functionally associated neurons and reveal an exquisite temporal organization in cell production within a precise spatially defined region of neuroepithelium.