Difference between the absolute infrared and blue rotation velocities at the midplane (thick dark red line) and the height where the thick-disc light starts to dominate (thick yellow line). Uncertainty intervals are indicated by the thin lines. The position at which x = 1.3 x_f is indicated with dashed lines. Only the galaxies for which the thick-disc light dominates a height range are shown. The curves have been smoothed with an 8″ window.