their emission lines are much narrower ($\vFWHM \ltsim 2000$) than is typical for broad-line objects (Osterbrock & Pogge 1985). NLS1s are extreme AGNs in other respects as well — their UV–optical properties correlate well with the Boroson & Green (1992) primary spectral eigenvector identified in principal component analysis. In other words, NLS1 classification correlates well with strong optical  emission and weak \[\]$\lambda\lambda4959$, 5007 emission (Boller, Brandt & Fink 1996). While the possible importance of such extreme AGNs has been recognized for two decades (Davidson & Kinman 1978), interest in NLS1s has increased recently as their unusual X-ray properties have come to light: they have unusually steep soft and hard X-ray spectra (Puchnarewicz et al. 1992; Boller, Brandt, & Fink 1996; Brandt, Mathur, & Elvis 1997) and undergo rapid non-linear variability (Boller et al. 1997). While rapid, large amplitude variability in the UV–optical has not been reported, existing data do not address well the relationship between the X-ray and long-wavelength variations. A good compilation of the observed properties of NLS1s is given by Taniguchi, Murayama, & Nagao (1999).

Possible explanations for the narrowness of the permitted lines include the following:

1.  NLS1s have more distant-than-normal BLRs (Mason, Puchnarewicz, & Jones 1996; 