Unwritten constitutional conventions have long been understood to be integral to the operation of Westminster parliamentary systems. The British legal scholar A.V. Dicey emphasized that "constitutional morality" supplemented legal rules in regulating the exercise of political power and limiting the discretion of government officials. The presence of a written constitution and judicially enforceable constitutional rules has sometimes been thought to render constitutional conventions superfluous, but these unwritten conventions have been common over the course of American history and have played an important role in defining the effective constitution of the polity. Constitutional law always threatens to displace constitutional morality, however, and unwritten conventions are often seen as in tension with the supremacy of the written text and the primacy of constitutional interpretation.