Tracing the bar properties with the quadrupole feature: correlation between the bar strength, S_bar and the strength of the quadrupole, S_quadrupole (left panel), and correlation between the bar length, R_bar and the length of the quadrupole, R_quadrupole (right panel), computed using all isolated thin+thick models and the TNG50 barred galaxies (see the legend). The black dash line denotes the best-fit straight line (of the form Y = AX + B) while the grey shaded region denotes the 5-σ scatter around the best-fit line. The properties of the bar (strength and extent) remain strongly correlated with the properties of the quadrupole structure (Pearson correlation coefficient, ρ > 0.75).