Article One of the United States Constitution
After much debate, the framers of the Constitution compromised and made population the basis of determining the number of seats (called apportionment) in the House of Representatives. It also used apportionment to determine the tax liability among the states. To accomplish this, the Constitution requires that a census be conducted every ten years. This is to determine the population of each state and of the nation as a whole. It also establishes a rule for who should and who should not be included  in the count. Because the Constitution would go into effect before the completion of a national census, it provides for a temporary apportionment of seats in the House of Representatives.