Constitutions became the foundation of modern republics by clearly defining the structure and limits of government. They established the rights and duties of citizens while outlining how leaders are chosen and held accountable. Written constitutions helped ensure stability by providing a legal framework that applied equally to all. They also protected fundamental freedoms such as speech, belief, and participation in governance. By setting rules that could not be easily altered by rulers, constitutions reinforced the idea that authority flows from the people rather than individuals or dynasties.