Four configurations for the quasar light source and foreground absorber. The continuum source (i.e., accretion disc) appears in red (or black when occulted), while the extended emission line region (ELR) is plotted in light blue. The absorber (green) may cover the continuum source, but only part of the ELR a), b), as assumed in the partial covering model. It may also cover part of the ELR, but not at all the continuum source c), or, more likely, cover the whole quasar source but in a non-uniform way, if internal structure is present (yellow areas correspond to regions inducing weaker absorption; the black dotted line delineates the ELR). In cases a) and b), absorption is stronger towards the continuum source than towards the ELR, whereas it is weaker in cases c) and d).