Histograms of spectral index for sources in PCCS2. The changes in the source populations with frequency are clearly visible, the lower frequencies are dominated by synchrotron sources and the higher frequencies by dusty ones. At the intermediate frequencies both source populations are discernible. Between the top two panels, there is a visible shift in the peak of the histogram. This is due to a steepening of the spectral indices of the radio sources. In each panel we give N, the number of sources in the histogram.