A constitution is the set of fundamental rules that creates a government, distributes power among its parts, limits that power, and usually protects basic rights. It answers questions ordinary laws depend on: who may make laws, by what procedure, and within what bounds.

Written and unwritten

Most countries have a codified (written) constitution: a single document with special legal status. The Constitution of the United States, drafted in 1787 and in force since 1789, is often described as the oldest written national constitution still in use.

A few countries, notably the United Kingdom, have an uncodified constitution. Its rules are found in many places rather than one document: statutes such as Magna Carta and the Bill of Rights of 1689, court decisions, and constitutional conventions. Conventions are unwritten practices treated as binding even though courts do not enforce them; for example, the monarch gives assent to bills passed by Parliament. “Unwritten” is a slight misnomer, since much of it is written, just not in one place. The UK also follows parliamentary sovereignty: Parliament can in principle make or unmake any law.

Separation of powers

In The Spirit of the Laws (1748), the French thinker Montesquieu argued that liberty is safest when the legislative, executive and judicial powers are held by different hands, because whoever holds all three can make tyrannical laws and enforce them tyrannically. Presidential systems separate the branches sharply and add checks and balances, such as a veto and legislative approval of appointments. Parliamentary systems partly fuse the executive and legislature, since the prime minister and cabinet sit in and answer to parliament, but they still keep the courts independent.

Judicial review

Judicial review is the power of courts to decide whether laws and government actions conform to the constitution, and to set aside those that do not.

• US example. In Marbury v. Madison (1803), Chief Justice John Marshall reasoned that the Constitution is supreme law and that “it is emphatically the province and duty of the judicial department to say what the law is.” The case established that federal courts may refuse to apply laws that conflict with the Constitution. Any court in the US system can apply this reasoning.
• The constitutional court model. Many countries instead give this power to a specialized constitutional court. Austria created one in 1920, following the ideas of the jurist Hans Kelsen, and Germany's Federal Constitutional Court, created in 1951, became highly influential.
• Weaker forms. Under the UK's Human Rights Act 1998, higher courts can declare a statute incompatible with protected rights, but Parliament decides whether to change it.

Supporters say judicial review protects rights and minorities from passing majorities. Critics raise what scholars call the counter-majoritarian difficulty: unelected judges overruling elected lawmakers on contested questions.

Amendment

Constitutions must be able to change, but not too easily. Most are entrenched, meaning they require more than an ordinary majority to amend. Under Article V of the US Constitution, an amendment is usually proposed by two-thirds of both houses of Congress and must be ratified by three-fourths of the states; as of the mid-2020s there have been 27 amendments. Ireland requires a referendum for every constitutional change. Germany's Basic Law contains an eternity clause that places certain principles, including human dignity and the federal structure, beyond amendment altogether, a response to the way the Weimar constitution was hollowed out in the 1930s.

Worked example: a law that seems to clash with the constitution

Imagine an invented republic whose constitution guarantees freedom of assembly. After a noisy protest, the legislature passes a law banning any public gathering of more than ten people without a permit, and gives the police full discretion to refuse permits without reasons. A community group is denied a permit for a vigil and goes to court. Here is how three systems might handle it.

A US-style system. The group can raise the constitutional claim in an ordinary court, which may refuse to apply the law if it conflicts with the constitution, following the reasoning of Marbury v. Madison. The case may climb to the supreme court, whose ruling then binds all lower courts.

A constitutional court system. An ordinary judge who doubts the law's validity refers the question to the specialized constitutional court, which may strike the law down or rule that it is valid only if interpreted narrowly, for example, if permits may be refused only for clear safety reasons.

A UK-style system. A higher court might declare the law incompatible with the right to assembly, but the law stays in force until Parliament decides whether to change it.

In each case, notice the separation of powers at work. The legislature made the law, the executive (the police) applied it, and the courts were asked to check both. Whether citizens think the courts should have the last word depends on how they weigh protecting rights against the counter-majoritarian difficulty.