The Constitution
Sixteenth Amendment
The 1913 amendment allowing Congress to tax income from any source without apportioning the tax among the states by population.
In full
The Supreme Court had struck down an 1894 income tax as an unapportioned direct tax. The amendment removed that obstacle and made the modern federal budget possible: income and payroll taxes now supply the great majority of federal revenue. Debates about what counts as income for constitutional purposes continue.
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A plain-language explanation for civic education, not legal advice. When in doubt, the official source named above controls.