ABSTRACT The quasar main sequence appears to be an incredibly powerful tool to organize the diversity in large samples of type-1 quasars but the most important physical parameters governing it are still unclear. Here, we investigate the origin of the broadening and of a defining feature of Population B sources: a strong redward asymmetry of the Balmer emission lines. We focus on a prototypical source, Fairall 9. Spectropolarimetric data of the Fairall 9 broad H β and H α profiles allowed for a view of the geometric and dynamical complexity of the line emitting regions. Measurements (1) provided evidence of rotational motion; (2) were helpful to test the presence of polar and equatorial scatterers, and their association with non-virial motions. However, we suggest that the polarization properties appear to be more consistent with a warped disc geometry induced by Lense–Thirring precession.