Article I, Section 2, Clause 3
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 three-fifths rule for counting slaves is often misunderstood. When the Constitutional Convention debated the issue of how to count population for the purposes of representation, the Southern delegates to the Convention would have been pleased if nonvoting slaves had been counted as full persons. That way, the Southern states would have had a greater representation in the House of Representatives. In contrast, some Northern delegates resisted counting slaves at all. Why, asked Elbridge Gerry, “shd. the blacks, who were property in the South, be in the rule of representation more than the cattle & horses of the North?” Among other things, counting slaves provided an incentive to import still more slaves.