Textbooks may be cited in court to assist in the interpretation of the law. They have no authority as a source of law but merely provide an expert opinion as to the current state of the law. There was formerly a convention that only the works of dead authors could be cited, but modern practice also allows citation of living authors. The Books of Authority, i.e. the works of Glanvil, Bracton, Littleton, Coke, and Blackstone, are treated as having the same authority as cases of the same period.