  Recent studies of black holes in the nuclei of both active and normal
galaxies have yielded relationships that permit a physical interpretation of
the principal components of the spectra of QSOs. It is shown that principal
component (or eigenvector) 1 (PC1) is driven predominantly by L/L{Edd}, and
principal component 2 (PC2) is driven by luminosity or accretion rate. This
results in a PC2 vs. PC1 diagram in which lines of constant black hole mass are
diagonal. Using a sample consisting of the low-redshift PG objects supplemented
by 46 radio-loud QSOs, it is shown that such a diagram effectively
distinguishes radio-loud from radio-quiet as well as demonstrating that both
narrow-line Seyfert 1s and broad absorption-line QSOs (BALQSOs) lie at the high
L/LEdd} extreme, though these two types of objects are well separated in the
PC2 direction. The very few radio-loud BALQSOs known fall in the region
expected to be populated by such objects. Finally, a simple picture that ties
together physical parameters and classification of AGN is presented.
