Sixteenth Amendment to the United States Constitution
When the framers of the Constitution were determining how the new government would be funded, they were trying to avoid a tax based on a person's income. In the Constitution, Article one, Sections 2 and 9 created the "rule of apportionment". On a practical basis this required Congress to tax the states based on each state's population instead of a tax on each person's income or property. For example, if a state made up four percent of the total population, they would be required to pay four percent of the total federal tax. The framers of the Constitution expected direct taxes to be used and defined the term broadly. However, they did not see that apportionment would cripple any attempt to collect direct taxes.