Constitutional law is a body of law that defines the role, powers and structure of the different entities within a state, namely the executive, parliament or legislature and the judiciary; as well as the basic rights of citizens and, in federal countries such as the United States and Canada, the relationship between the central government and state, provincial or territorial governments.
Not all nation states have codified constitutions, although all those states have a jus commune, or law of the land, which can consist of a variety of imperative and consensual rules. These may include customary laws, conventions, statutory laws, laws created by judges or international rules and regulations. The constitutional law deals with the fundamental principles by which the government exercises its authority. In some cases, 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 put limits on what the government can do, such as prohibiting the arrest of an individual without sufficient cause.
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 was created. Other constitutions, notably that of the United Kingdom, are based largely on unwritten rules known as constitutional conventions; their status within constitutional law varies, and the terms of the conventions are in some cases strongly contested.