Abstract
Some interesting aspects of the structure and stoichiometry of YBa2Cu3O7+δ and other high-Tc oxide superconductors are discussed. The nature of Cu and the importance of the Cu3+/Cu2+ ratio in these copper oxides are pointed out. Marginal metallicity of the non-superconducting phase in these oxides as an important factor in the mechanism is indicated. Effect of Y substitution in YBa2Cu3O7+δ by other rare earths is commented upon.