<P> Constitutional law is the body of law which defines the role, powers, and structure of different entities within a state, namely, the executive, the parliament or legislature, and the judiciary; as well as the basic rights of citizens and, in federal countries such as India and Canada, the relationship between the central government and state, provincial, or territorial governments . </P> <P> Not all nation states have codified constitutions, though all such states have a jus commune, or law of the land, that may consist of a variety of imperative and consensual rules . These may include customary law, conventions, statutory law, judge - made law, or international rules and norms . Constitutional law deals with the fundamental principles by which the government exercises its authority . In some instances, these principles grant specific powers to the government, such as the power to tax and spend for the welfare of the population . Other times, constitutional principles act to place limits on what the government can do, such as prohibiting the arrest of an individual without sufficient cause . </P> <P> In most nations, such as the United States, India, and Singapore, constitutional law is based on the text of a document ratified at the time the nation came into being . Other constitutions, notably that of the United Kingdom, rely heavily on unwritten rules known as constitutional conventions; their status within constitutional law varies, and the terms of conventions are in some cases strongly contested . </P> <P> Constitutional laws may often be considered second order rule making or rules about making rules to exercise power . It governs the relationships between the judiciary, the legislature and the executive with the bodies under its authority . One of the key tasks of constitutions within this context is to indicate hierarchies and relationships of power . For example, in a unitary state, the constitution will vest ultimate authority in one central administration and legislature, and judiciary, though there is often a delegation of power or authority to local or municipal authorities . When a constitution establishes a federal state, it will identify the several levels of government coexisting with exclusive or shared areas of jurisdiction over lawmaking, application and enforcement . Some federal states, most notably the United States, have separate and parallel federal and state judiciaries, with each having its own hierarchy of courts with a supreme court for each state . India, on the other hand, has one judiciary divided into district courts, high courts, and the Supreme Court of India . </P>

Who determines if a law is constitutional or not