Spectral index between 1.4 and 5 GHz vs. in-band spectral index at 5 GHz. From the left, F sample, Q sample, and S sample. The colors indicate the classification, from left to right: black is for F objects, blue for the Q sample, and red for the S sources. Circles indicate compact sources, squares intermediate sources and triangles extended sources. The dashed black line indicates the 1:1 ratio, the dashed-dotted red lines separate the region of flat-spectrum sources (α < 0.5) from that ofsteep-spectrum objects.