Temporal evolution of vertical-to-radial velocity dispersion (σ_zz/σ_RR) calculated within R_b/p, (σ_zz/σ_RR)²(t; R ≤ R_b/p) for thin (in blue), thick (in red), and total (thin+thick) disc (in black) particles, for all thin+thick-disc models. Left panels show the evolution for the rthickS models, whereas middle panels and right panels show the evolution for the rthickE and rthickG models, respectively. The thick-disc fraction (f_thick) varies from 0.1 to 1 (top to bottom), as indicated in the left-most panel of each row. The vertical maroon dotted line denotes the onset of bar formation (τ_bar), while the vertical magenta dotted line denotes the onset of buckling instability (τ_buck; for details, see text).