Three phase-space spirals (labelled A, B, and C) inhabiting three different gravitational potentials. Top panel: vertical force per mass (K_z = −∂Φ/∂z) as a function of height for the respective gravitational potentials. Bottom panel: three spirals in the (z, w)-plane, where their winding time (t) and initial angle () are chosen such that the spirals have the same phase () and phase derivative () when their vertical energy is equal to E_z = Φ(800 pc).