Some of the sources identified in the early X-ray surveys of the sky were subsequently found to be either Seyfert galaxies or galaxies with some Seyfert-like characteristics. The latter sources were characterized by the same high-excitation emission lines as Seyfert galaxies, but with lower luminosities than typical of Seyferts (Ward et al. 1978). These galaxies, which are sometimes called ``narrow-line X-ray galaxies'' (NLXGs), are Seyfert galaxies whose optical spectra are heavily reddened and extinguished by dust within the galaxy.

Adapted from B.M. Peterson An Introduction to Active Galactic Nuclei, Cambridge University Press, (1997)