Upper panel: Midplane values of Q_3D, ⋆ (filled symbols) and Q_3D, gas (empty symbols) as functions of the midplane star-to-gas density ratio, ξ, for different values of the star-to-gas squared velocity dispersion ratio, μ. The horizontal dashed line indicates the instability threshold, Q_3D = 1. Lower panel: Extent of the unstable strip above the midplane for the stellar (z_inst, ⋆, normalised to z_half, ⋆; filled symbols) and gaseous (z_inst, gas, normalised to z_half, gas; empty symbols) components as functions of ξ for different values of μ. In both panels, we assume that Q_gas = 0.6.