The clause established the basis for the U.S. Census and the apportionment of seats in the House of Representatives, including the “Three-Fifths Compromise,” later nullified by the 14th Amendment.
The actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States…
— Original Text, 1787
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 actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they shall by Law direct.