Distribution of SMBHs in mass and redshift inferred from the SDSS DR7 quasar catalog (left panel) and histograms of the distribution in redshift (middle) and mass (right). The one-dimensional distributions are the projections of the two-dimensional distribution on the mass and redshift axes. We assume that the mass distribution of primary SMBHs in hypothesized binaries has the same shape as for the SMBHs in SDSS quasars. The color bar marks the number of quasars.