An Apportionment Bill is an act passed by the Congress of the United States after each decennial census to determine the number of members which each state shall send to the United States House of Representatives. The number of the members of the first House was 65. Following the first decennial census the House was enlarged to approximate the Constitutional maximum of "Number of Representatives shall not exceed one for every thirty Thousand". As the House would, at this ratio, have become much larger than today's 435 members, the ratio has been raised after most censuses, as will be seen from the accompanying table.
The same term is applied to the acts passed by the state legislatures for making their electoral districts equal in population. Such acts are usually passed at decennial intervals, more often after the federal census, but the dates may vary in different states. As per Reynolds v. Sims, the electoral districts so formed are expected to be equal in proportion to the number of inhabitants; but districts drawn for the benefit of one or more political party are not prohibited as long as they are equal in population (see gerrymander).
If a state has received an increase in the number of its Representatives and its legislature does not pass an apportionment bill before the next congressional election, the votes of the whole state elect the additional members on a general ticket and they are called "Representatives at-large."