Constitution of the United States, Article. I., Section. 2...

Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons

The "direct Taxes" mentioned above were to be in the form of property taxes and a head tax.