The U.S. officially adopted the Constitution in 1788, after it was signed on September 17, 1787, by delegates at the Constitutional Convention in Philadelphia. The Constitution was ratified by nine states, which allowed it to take effect in March 1789. This document established the framework of the federal government, delineated the powers of its branches, and outlined the rights of citizens. Its adoption marked a significant shift from the Articles of Confederation, leading to a stronger central government and providing a lasting foundation for American democracy and law.