Composite LBT K_S spectra for N and W + w quasars. Top panel: Median LBT K_S spectra for the X-ray normal and the X-ray weak subsamples. The colour-coding is the same as in Fig. 3. Fluxes are normalised by their value at 5100 Å. Middle panel: Number of spectra contributing to each spectral channel, following the same colour-coding. Bottom panel: Ratio of the N spectrum to the W+w spectrum.