Temporal evolution of the bar strength, S​_bar, for the thin-disc (upper panels), thick-disc (middle panels), and total (thin+thick) disc stars (lower panels) for all thin+thick disc models with varying f_thick values (see the colour bar). Left panels: bar strength evolution for the rthickS models, and the middle and right panels: evolution of the bar strength for the rthickE and rthickG models, respectively. The thick-disc fraction (f_thick) varies from 0.1 to 0.9 (with a step size of 0.2), as indicated in the colour bar. The solid blue lines in the bottom row denote the three thick-disc-only models (f_thick = 1), and the solid red line in the top middle panel denotes the thin-disc-only model (f_thick = 0). For details see text.