Blazars have been defined as extragalactic objects characterized by rapid optical variability, high optical polarization, large X-ray and -ray luminosities and a variable, flat-spectrum, superluminal radio core. The class of blazars includes the BLLs which have no or weak emission lines and the HPQs with strong, broad emission lines similar to those of normal QSOs.
The spectral energy distribution of BLLs discovered in X-ray and radio surveys differ significantly, leading to the subclassification of BLLs into X-ray selected and radio-selected objects (XBLs and RBLs respectively); this has subsequently been supplanted by a new classification ``High energy peaked BLLs'' (HBLs) and ``Low energy peaked BLLs'' (LBLs) (). Generally, XBLs tend to be HBLs and exhibit less extreme properties than RBLs which are usually LBLs. Studies of statistically complete samples of BLLs show that the observed dichotomy between XBLs and RBLs is an observational bias (; ).
Many of the observed properties of blazars can be reasonably understood if they are interpreted to be radio galaxies with their relativistic jet pointing toward us, BLLs and HPQs being associated with FR Is (or LERGs) and FR IIs respectively.
PKS1413+135 is a puzzling object; while all other known BLLs have been found to be hosted by normal E galaxies (), it is in an edge-on spiral () with a heavily obscured nucleus ().