Bar strength (from Díaz-García et al. 2016a) as a function of spiral strength, measured from m = 2 Fourier amplitudes (Eq. (11)) (left), from tangential-to-radial forces (Eq. (9)) (middle), also including the correction for the halo dilution (Eq. (10)) (right). Different types of spirals are plotted with different symbols, as indicated in the legend. The red dashed line shows the linear fit to the cloud of points (in green and black we show the fit for flocculent and grand-design+multi-armed, respectively).