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tokens: of the system or the effects of anisotropic line emission, are subsumed into the constant f . Thus, f is expected to be different for every individual AGN, but should be approximately constant for every emission line in a given AGN assuming similar geometries and dynamics of the line-emitting gas. In every case where the lags from multiple emission lines can be measured in a single object, it is found that , as expected from Equation ( 1 ), suggesting that this is the case (Peterson Wandel 1999 , 2000 ; Kollatschny 2003 ; Peterson et al. 2004 ; Bentz et al. 2010b ). It is worth reminding the reader that the scale factor f depends on which parameter is used to characterize the emission-line width, as we discuss in Section 4 . The scaling factor f can be determined for an AGN if there is an independent measurement of the black hole mass. Unfortunately, there are few cases where the black hole radius of influence is large enough that either stellar or gas dynamical modeling can also be used. At the present time, there are stellar dynamical masses (Davies et al. 2006 ; Onken et al. 2014 ) and gas dynamical masses (Hicks Malkan 2008 ) for NGC 3227 and NGC 4151; these are useful for comparison purposes, but it would be unwise to attempt to calibrate the entire reverberation-based mass scale on only two objects. Instead, one of the well-known correlations between central black hole mass and properties of the host galaxies can be used. The first of these to be used to calibrate the AGN black hole mass scale was the correlation between black hole mass and host galaxy luminosity (Magorrian et al. 1998 ) by Laor ( 1998 ). More recently, calibration of the AGN black hole mass scale has been based on the strong correlation between the black hole mass and the velocity dispersion of host-galaxy bulge, the M BH – σ * relationship, which applies to both quiescent (Ferrarese Merritt 2000 ; Gebhardt et al. 2000a ; Tremaine et al. 2002 ; Gültekin et al. 2009 ; McConnell et al. 2011 ; McConnell Ma 2013 ) and active galaxies (Gebhardt et al. 2000b ; Ferrarese et al. 2001 ; Nelson et al. 2004 ; Onken et al. 2004 ; Dasyra et al. 2007 ; Woo et al. 2010 , 2015 ; Graham et al. 2011 ; Park et al. 2012 ; Grier et al. 2013a ; Batiste et al. 2017 ). With σ * measurements now available for ∼30 AGNs from the reverberation-mapping database (Woo et al. 2015 ), an ensemble average can be computed by comparing the predicted masses from the M BH – σ * relationship with the observed VPs, using the line dispersion to characterize the line width. Using this prescription, black hole masses have been measured for ∼60 AGNs using reverberation mapping (see Bentz Katz 2015 for an up-to-date compilation). An important result from reverberation mapping is the observed “ R – L ” relationship between the size of the BLR and the AGN luminosity (Wandel et al. 1999 ; Kaspi et al. 2000 , 2005 ; Bentz et al. 2006b , 2009a , 2013 ). This R – L relationship allows us to bypass