Unlike treaty law, customary international law is not written. To prove that a certain rule is customary one has to show that it is reflected in state practice and that there exists a conviction in the international community that such practice is required as a matter of law. In this context, "practice" relates to official state practice and therefore includes formal statements by states. A contrary practice by some states is possible because if this contrary practice is condemned by other states or denied by the government itself the original rule is actually confirmed.