The federal government collected $2.2 trillion in income taxes in 2021. Before 1913, that would have been unconstitutional.
The Constitution originally required direct taxes to be divided among states by population. If Congress wanted to raise $100 million, California would pay based on its share of the national population, not its share of national wealth. This made a progressive income tax impossible—you couldn't tax rich states more without violating the apportionment rule. The 16th Amendment, ratified in 1913, bypassed this requirement by giving Congress power to tax income "from whatever source derived, without apportionment." Today, the top 50 percent of earners pay 97 percent of all federal income taxes. This progressive structure only works because the 16th Amendment freed income taxes from the apportionment straightjacket.