Spectral fits of (from top to bottom) Mrk 335, Mrk 110, Mrk 509, and Ark 564 in the wavelength ranges considered, with specific fluxes measured in units of 10⁻¹⁵ erg s⁻¹ cm⁻² Å⁻¹ and the wavelength measured in Å. A lower box describing the residuals is associated with each spectral fit, with the wavelength expressed in units of radial velocities (km s⁻¹). The original spectrum is plotted with a black line and the modeled spectrum is plotted in magenta. Each additional line component is plotted with a different colour, according to the legend. Dashed vertical lines represent the rest-frame wavelengths of the main lines of the selected spectral range. Left: λ1320-1450 Å range. Middle left: λ1450-1700 Å range. Middle right: λ1800-2100 Å range. Right: λ4430-5300 Å range.