  Blazars are currently separated into BL Lacertae objects (BL Lacs) and flat
spectrum radio quasars (FSRQ) based on the strength of their emission lines.
This is done rather arbitrarily by defining a diagonal line in the Ca H&K break
value -- equivalent width plane, following Marcha et al. We readdress this
problem and put the classification scheme for blazars on firm physical grounds.
We study ~100 blazars and radio galaxies from the Deep X-ray Radio Blazar
Survey (DXRBS) and 2 Jy radio survey and find a significant bimodality for the
narrow emission line [OIII] 5007. This suggests the presence of two physically
distinct classes of radio-loud AGN. We show that all radio-loud AGN, blazars
and radio galaxies, can be effectively separated into weak- and strong-lined
sources using the [OIII] 5007 -- [OII] 3727 equivalent width plane. This plane
allows one to disentangle orientation effects from intrinsic variations in
radio-loud AGN. Based on DXRBS, the strongly beamed sources of the new class of
weak-lined radio-loud AGN are made up of BL Lacs at the ~75 per cent level,
whereas those of the strong-lined radio-loud AGN include mostly (~97 per cent)
quasars.
