Distribution of the quasar sample (excluding radio-loud and BAL sources) in a Γ₁–Γ₂ plane, where Γ₁ and Γ₂ are the slopes of a power law in the log ν–(log νL_ν) plane, at 0.3–1 μm and 1450–3000 Å, respectively. The red stars represent Γ₁–Γ₂ values for the intrinsic mean SED for quasars of Richards et al. (2006) reddened by the presence of dust (extinction law of Prevot et al. 1984) with increasing E(B-V), ranging from 0 to 0.5. We selected the sources inside the red circle of centre E(B − V)=0 and radius ∼1 (corresponding to E(B − V)=0.1), characterised by the minimum extinction. Other possible explanations to the deviation of points from the locus of blue quasars are, for example, the presence of host-galaxy contamination and, trivially, wrong photometric measurements.