The Main Sequence of quasars: the taming of the extremes
  The last few years have seen the confirmation of several trends associated
with the quasar main sequence. The idea of a main sequence for quasars is
relatively recent, and its full potential for the observational classification
and contextualization of quasar properties has yet to be fully exploited. The
main sequence drivers are discussed in terms of the properties of extreme
objects. We briefly summarize developments that constrain the viewing angle of
the accretion disk in a particular class of quasars (extreme Population B,
radiating at low Eddington ratio), as well as inferences on the chemical
composition of the broad line emitting gas, and on the nature of radio emission
along the quasar main sequence.
