Correlation between the two methods of measuring the quadrupole extent: R_quadrupole, measured solely from the amplitude of the m = 4 Fourier coefficient of the mean radial velocity ⟨V_R⟩, and R_{quadrupole, φ4}, measured solely from the constancy of the phase-angle, φ₄ of the mean radial velocity ⟨V_R⟩, as a function of bar age (t_bar-age, see the colour bar). Both the quantities, R_quadrupole and R_{quadrupoleφ4}, are normalised by the mean disc scale length, ⟨R_d⟩. For details, see the text. The dotted straight line denotes the 1:1 relation. The colour bar denotes the age of the bar.