"Soft" effective spectral index (α_S) plotted against the "hard" effective spectral index (α_H) for (a) radio‐quiet quasars, and (b) radio‐loud quasars. Radio‐loud flat‐spectrum radio sources are identified by circles, steep radio spectrum sources by squares. High‐redshift quasars (z > 2.2) are shown with filled symbols. Three‐point spectra illustrate the radically different spectral shapes in different parts of the diagrams.