Abstract
Within a simple and natural tight-binding model, we show that the increase in critical temperature from La2xSrxCuO4 (or YBa2Cu3O7δ with δ0.5) to YBa2 Cu3 O7 can easily be explained by the increase in density of states due to the CuO chains present in YBa2 Cu3 O7. We discuss the effects of moving the Fermi energy and shifting the chain band with respect to the plane bands.