We offer a broad review of Balmer line phenomenology in type 1 active
galactic nuclei, briefly sum- marising luminosity and radio loudness effects,
and discussing interpretation in terms of nebular physics along the 4D
eigenvector 1 sequence of quasars. We stress that relatively rare, peculiar
Balmer line profiles (i.e., with large shifts with respect to the rest frame or
double and multiple peaked) that start attracted attentions since the 1970s are
still passable of multiple dynamical interpretation. More mainstream objects
are still not fully understood as well, since competing dynamical models and
geometries are possible. Further progress may come from inter-line comparison
across the 4D Eigenvector 1 sequence.Comment: Accepted for publication in Astrophysics and Space Science, Special
Issue on Line Shifts in Astrophysics and Laboratory Plasm
