Which observational differences are still left between Seyfert 1 and Seyfert 2 nuclei?

Reviews the different observational properties used, up to now, to discriminate between Seyfert 1 and Seyfert 2: emission lines widths, Fe II lines, intensity ration (OIII) 500.7 nm/H beta , radio, infrared, visible, UV and X-emission. Most of these parameters exhibit a more or less continuous distribution from the less powerful Seyfert 2 up to the Seyfert 1 nuclei (except in the radio range). (0 refs).