Constitution is a statement outlining the agreed basic principles of organizations. The organizations that make constitutions range from national governments to private clubs. A constitution establishes the structure and purposes of the organization and the rights of its citizens or members. It also defines the powers of officers, how they are selected, and how long they can stay in office. Constitutions may also be called articles of union or charters (see Charter).
In Western political philosophy, the principles of constitutional government often have been based on a belief in a higher law. Higher law is a body of universal principles of right and justice. It is superior to detailed, everyday law. In modern democracies, a constitution’s function is to put everyone—including the rulers—under law.
Government constitutions may be written or unwritten. The Constitution of the United States is an example of a written constitution. The United Kingdom has an unwritten constitution that includes laws passed by Parliament and court decisions. It also includes customs and traditions regarding the powers of the monarch, Parliament, and the courts. Many parts of the British constitution were taken from written documents, such as Magna Carta. However, the constitution itself has never been written out in a single document. The British constitution can be modified by Parliament. 
Most modern governments have constitutions based on a single document. In most democracies, the written constitution can be changed only by a special process, such as a special election. Such amending procedures reflect the belief that a constitution should deal with basic principles. For that reason, special deliberation should be required to modify or replace these principles.
Many countries now have undemocratic or military governments. In these governments, a constitution can be changed by fiat (a command or decree) of the ruling group. In such countries, a constitution is more a statement of purpose than a statement that defines government powers or establishes citizens’ rights.
In actual operation, constitutions in most democratic countries are unwritten. This means that the formal document is not the only governing element. The ways that various governmental bodies interpret the constitution may be equally important. Sometimes, they are even dominant. In the United States, for instance, courts have a power called judicial review. Judicial review allows courts to declare acts of government unconstitutional if the acts conflict with the Constitution. Also, most countries have important nonlegal rules that do not come from the written constitution or from court interpretation. If these nonlegal rules are an essential part of the system of government, they are part of the “constitution” in the broad sense of the term.
