Why is the U.S. Constitution considered the oldest written national constitution still in use?

The United States Constitution is recognized as the oldest written national constitution still in effect because it has remained the supreme law of the land since it entered into force in 1789.12

Its longevity is often attributed to a balance between "precise" and "vague" language, which has allowed the document to remain adaptable to changing historical circumstances while maintaining its core authority. The founders intentionally left certain phrases open for future interpretation, ensuring the document's relevance across generations.31

Furthermore, the Constitution is a "rigid" document, meaning it requires a formal, challenging amendment process—typically involving supermajorities in Congress and ratification by the states—to be modified. While this rigidity provides stability, the document has proven to be a dynamic framework, having been amended 27 times to address evolving societal needs, such as the expansion of suffrage and the protection of civil liberties.423