Temporal evolution of the bar strength S​_bar for the six thick-disc-only models with a varying vertical scale height. For details, see the text. The solid line denotes the rthickE models, the dashed line denotes the rthickG models, and the dotted line denotes the rthickS models. Different line colours denote models with different scale-height values. With increasing vertical scale height, the models progressively become bar stable. The horizontal line denotes S​_bar = 0.2 and is used as a (operational) limit for the onset of bar formation.