This article presents an account of the contours of Parliament’s law-making powers including an explanation of their limits and how those limits arise. The argument is based on HLA Hart’s claim that legal validity ultimately depends on the requirements of a social rule, the ultimate rule of recognition, that binds the officials of a legal system to enforce laws that conform to certain criteria. Section 2 outlines the way in which rules of recognition can be indeterminate and argues that the (...) rule of recognition in the United Kingdom’s legal system is in fact indeterminate on the question of the scope of Parliament’s law-making power. Section 3 explores the effects of indeterminacy in the rule of recognition, arguing that indeterminate social rules develop according to rule users’ understanding of the point of the rule, and that the rule of recognition will therefore develop according to the judiciary’s understanding of the value or values served by the according law making power to Parliament. (shrink)