  The main sequence offers a method for the systematization of quasar spectral
properties. Extreme FeII emitters (or extreme Population A, xA) are believed to
be sources accreting matter at very high rates. They are easily identifiable
along the quasar main sequence, in large spectroscopic surveys over a broad
redshift range. The very high accretion rate makes it possible that massive
black holes hosted in xA quasars radiate at a stable, extreme
luminosity-to-mass ratio. After reviewing the basic interpretation of the main
sequence, we report on the possibility of identifying virial broadening
estimators from low-ionization line widths, and provide evidence of the
conceptual validity of redshift-independent luminosities based on virial
broadening for a known luminosity-to-mass ratio.
