  The phenomenology of gamma-ray bright blazars can be accounted for by a
sequence in the source power and intensity of the diffuse radiation field
surrounding the relativistic jet. Correspondingly, the equilibrium particle
distribution peaks at different energies. This leads to a trend in the observed
properties: an increase of the observed power corresponds to: 1) a decrease in
the frequencies of the synchrotron and inverse Compton peaks; 2) an increase in
the ratio of the powers of the high and low energy spectral components. Objects
along this sequence would be observationally classified respectively as high
frequency BL Lac objects, low frequency BL Lac objects, highly polarized
quasars and lowly polarized quasars. The proposed scheme is based on the
correlations among the physical parameters derived in the present paper by
applying to 51 gamma ray loud blazars two of the most accepted scenarios for
the broad band emission of blazars, namely the synchrotron self--Compton and
external Compton models, and explains the observational trends presented by
Fossati et al. (1998) in a companion paper, dealing with the spectral energy
distributions of all blazars. This gives us confidence that our scheme applies
to all blazars as a class.
