Temporal evolution of the strength of the quadrupole, S_quadrupole and the extent of the quadrupole, R_quadrupole (normalised by the mean disc scale length, ⟨R_d⟩), for thin+thick models with different f_thick values (see the colour bar), as a function of bar age (t_{bar − age}). Left panels: rthickS models. Middle and right panels: rthickE and rthickG models, respectively. The strength and extent of the quadrupole for the model sim6 are shown in blue lines (see the left panels).