321 Si IV “centroids” (or rather median spectra) depicting the different spectral shapes that were found for IBs. Three spectra are plotted in different colors in each panel to save space, but during the classification, all of them were treated independently. The numbers indicate how many spectra were classified into a given centroid. The red dotted lines indicate the wavelengths of line center and of the Ni II blend at 1393.33 Å.