  The dynamic evolution of binary systems of supermassive black holes (SMBH)
may be a key factor affecting a large fraction of the observed properties of
active galactic nuclei (AGN) and galaxy evolution. Different classes of AGN can
be related in general to four evolutionary stages in a binary SMBH: 1) early
merger stage; 2) wide pair stage; 3) close pair stage; and 4) pre-coalescence
stage. This scheme can explain a variety of properties of AGN: radio and
optical luminosity differences between different classes of AGN, long-term and
short-term variability, quasi-periodic nuclear flares, recurrent formation of
relativistic outflows in AGN and their apparent morphology and kinematics.
