Article One of the United States Constitution
Originally, the population of each state and of the nation as a whole was determined by adding to the whole number of free Persons, three-fifths the number of all other persons (slaves), but excluding non-taxed Native Americans. This Constitutional rule was  known as the three-fifths compromise. It was used to determine the number of Representatives in the House. Larger states contributed more money and would have more seats in the House of Representatives.