... The picture shows sketches of the optical and ultraviolet (top, left) and X-ray (top, right) spectra of a typical quasar or AGN (Active Galactic Nucleus). These spectra are full of sharp features due to the interaction of atoms with light ('atomic features'). They can be high (= bright) points, called 'emission lines' or low (= dark) points, called 'absorption lines' . ... Broad Emission Lines ("BELs") [seen in 100% of AGN], . ... Broad Absorption Lines ("BALs") [see in only 10% of AGN], . ...