The 16th Amendment, establishing the legal basis for a federal income tax, was ratified in 1913.

The 16th Amendment, which forms the basis of all federal income tax legislation, gives Congress the power to "lay and collect taxes on incomes, from whatever source derived." Sensitive to issues of taxation by a federal power, the founding fathers wrote in Article I, Sections 2 and 9 of the Constitution that the federal government could impose no tax unless it was apportioned among the states according to population. The 16th Amendment served to settle controversy over the legality of federal income tax that had led to two Supreme Court cases with disparate outcomes.