Phase-space spiral represented as a line graph, shown in solid blue. Two points, labelled A and B, are connected to the origin point via dashed black lines. Points A and B are both associated with a total vertical energy: E_z, A = w²/2 is given by the vertical velocity where the spiral passes through the Galactic mid-plane; and E_z, B = Φ(z) is given by gravitational potential at a point of the spiral with zero velocity. In the limit of high winding, the difference E_z, A − E_z, B would approach zero from above.