  The average Spectral Energy Distributions (SED) of different classes of
blazars from the radio to the gamma-ray band are studied adopting a
"complete-sample" approach. The SED are globally similar, despite systematic
differences in specific frequency ranges, showing two broad peaks. However, i)
the first and second peak occur in different frequency ranges for different
objects, with a tendency for the most luminous sources to peak at lower
frequencies, ii) the ratio between the two peak frequencies *seems to be
constant*, iii) the luminosity ratio between the high and low frequency
component increases with the bolometric luminosity.
