FWHM of a line in the mean spectrum or a single-epoch spectrum and  is the FWHM in the rms spectrum. It is common to use  as the line-width measure for determining black hole masses from reverberation data — it is intuitatively a good choice as it isolates the gas in the BLR that is actually responding to the continuum variations. As noted previously, the strong and strongly variable broad emission lines can be hard to isolate as they are blended with other features. In the rms spectra, however, the contaminating features are much less of a problem because they are generally constant or vary either slowly or weakly and thus nearly disappear in the rms spectra.

Since the goal is to measure a black hole mass from a single (or a few) spectra, we must use a proxy for . Here we will attempt to determine if either  or  in a single or mean spectrum can serve as a suitable proxy for ; we know [*a priori*]{} that there are good, but non-linear, correlations between  and both  and . It therefore seems likely that either  or  could be used as a proxy for .

Investigation of the relationship among the 