The broad topic of constitutional law deals with the interpretation and implementation of the United States Constitution. 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. In most nations, including the United States, constitutional law is based on the text of a document ratified at the time the nation came into being.
The text of the U.S. Constitution is marked by four characteristics: a delegation of power, in which the duties and prerogatives of the executive, legislative, and judicial branches are delineated by express constitutional provisions; a Separation of Powers, in which the responsibilities of government are divided and shared among the coordinate branches; a reservation of power, in which the sovereignty of the federal government is qualified by the sovereignty reserved to the state governments; and a limitation of power, in which the prerogatives of the three branches of government are restricted by constitutionally enumerated individual rights, Unenumerated Rights derived from sources outside the text of the Constitution, and other constraints inherent in a democratic system where the ultimate source of authority for government action is the consent of the people.
The U.S. Supreme Court is the authority on all constitutional law issues. Only a Constitutional Amendment approved by three-fourths of the states can override a decision made by the Supreme Court. Wherever the process of constitutional interpretation goes on, it must be guided by some more or less articulate theory about the extraction of meaning from constitutional language. The interpreter cannot merely insist that the constitution means whatever he wants it to mean, or at least he cannot admit such an approach to the interpretative process.