The fundamental rules that establish and limit government are collected in a constitution. It defines the branches of power, the rights of citizens, and the procedures for making ordinary law.

Constitutions may be written single documents or unwritten collections of statutes, conventions, and judicial decisions. They typically claim supremacy over ordinary legislation. Amendment procedures are usually more demanding than ordinary lawmaking, so that the basic framework remains relatively stable.

Constitutional design shapes political competition: presidential versus parliamentary systems, federal versus unitary structures, and the strength of judicial review all flow from constitutional choices. When political actors treat the constitution as binding even when it frustrates their immediate goals, the system gains legitimacy and predictability. When they treat it as optional, constitutionalism itself erodes.

A constitution is only as strong as the willingness of powerful actors to abide by it.