  It is argued that the dominant feature of the phase diagram of the high $T_c$
cuprates is the crossover to the pseudogap phase in the energy (temperature)
region $E (T^*)$. We argue that this scale is determined by the effective
antiferromagnetic interaction which we calculate to be $J_{eff} = J_{\rm
superexchange} - xt$ where $x$ is the hole percentage and $t$ the hopping
integral.
