The Sixteenth Amendment to the United States Constitution allows Congress to levy an income tax without apportioning it among the states on the basis of population. It was passed by Congress in 1909 in response to the 1895 Supreme Court case of Pollock v. Farmers' Loan & Trust Co. The Sixteenth Amendment was ratified by the requisite number of states on February 3, 1913, and effectively overruled the Supreme Court's ruling in Pollock.

The Constitution of the United States is the supreme law of the United States. It superseded the Articles of Confederation, the nation's first constitution, on March 4, 1789. Originally including seven articles, the Constitution delineates the national frame and constrains the powers of the federal government. The Constitution's first three articles embody the doctrine of the separation of powers, in which the federal government is divided into three branches: the legislative, consisting of the bicameral Congress ; the executive, consisting of the president and subordinate officers ; and the judicial, consisting of the Supreme Court and other federal courts. Article IV, Article V, and Article VI embody concepts of federalism, describing the rights and responsibilities of state governments, the states in relationship to the federal government, and the shared process of constitutional amendment. Article VII establishes the procedure subsequently used by the 13 states to ratify it. The Constitution of the United States is the oldest and longest-standing written and codified national constitution in force in the world.