In the United States Constitution, the difference between indirect and direct taxes was important enough to require a Constitutional amendment in order for the federal government to levy an income tax. This was the Sixteenth Amendment which was ratified in 1913. Before the Sixteenth Amendment, any direct tax levied by the federal government had to be apportioned among the states by population. As apportionment by population proved to be virtually impossible, levying direct taxes was prevented by this article of the Constitution until it was changed in 1913. So the federal government relied on indirect taxes such as tariffs and Duties on imported goods and materials.