Location of the Sérsic model family, as well as most other popular spherical toy models, in the (TGC, C_S) and the (TGC, TGC_3D) planes. The models form banana-shaped trails in the former plane, whereas they are located on an almost perfect one-to-one relation in the latter plane. This shows that the Spitzer concentration index is a poor indicator of the intrinsic 3D concentration, whereas the TGC is a very accurate indicator.