Lower panels: plots of effective carrier density vs. doping in LSCO (left) and YBCO (right). The open squares are obtained from integrating the optical conductivity to $650\textrm{ cm}^{-1}\approx 80$~meV (lower points) and to $10,000\textrm{ cm}^{-1}\approx1.25$~eV (upper points). The filled circles show the density of carriers determined from the Hall coefficient $R_{\rm H}$ measured at 250 K (lower points) and at 720 K (upper points). Upper panels: estimates of the effective mass. From Padilla {\it et al.} \cite{padi05}.