Apportionment or apportionment by population is a concept in United States law of allocating representatives or taxes among the US states. Apportionment is based on the United States Census taken every ten years. Seats in the United States House of Representatives are then apportioned among the states. Each state gets a number of seats based on its population (the minimum is one seat). Congress enacted new apportionment legislation following every census until the mid-20th century. of the United States Constitution requires that direct taxes be apportioned among the states by population. Apportionment by population proved to be a nearly impossible, uneven and unfair requirement for collecting taxes since the states had different populations. This was overturned by the Sixteenth Amendment to the United States Constitution which removed the apportionment requirement from Congress imposing direct taxes.