BL Lacertae (BL Lac or S4 2200+420) was known to be variable in the optical from as early as 1929, and because of its stellar appearance it was originally thought to be a VARIABLE STAR. It is located in the constellation Lacerta, the lizard (RA (J2000)=22h 02' 43.29'', DEC (J2000)=+42° 16' 39.98''). Observations in the late 1960s showed it to have highly variable radio emission as well.