Contours: density of SDSS emission-line galaxies in the optical diagnostic planes (adapted from Kewley et al. 2006). Galaxies below the curved solid lines are star-forming galaxies. In the left diagram, the dashed curve marks the transition from composite galaxies to AGN; in the middle and right panels, the oblique line separates LINERs from Seyferts. The circles are the 3CR sources. These diagrams reveal the existence of a class of extremely low excitation galaxies, i.e. the three filled red circles corresponding to the objects with the lowest  [O   III] /Hβ ratios.