Example of the Γ₁ − Γ₂ distribution for the XXL quasar sample, where Γ₁ and Γ₂ are the slopes of a power law in the log(ν)−log(νL_ν) plane in the 0.3–1 μm and 1450–3000 Å intervals, respectively (see Sect. 7.1). The stars represent the Γ₁ − Γ₂ values of the quasar SED by Richards et al. (2006) with increasing dust reddening (following the extinction law of Prevot et al. 1984), with E(B − V) in the range 0–0.3. We selected all the quasars inside the blue circle (i.e. with minimum host-galaxy and dust reddening contamination).