The Apportionment Bill is an act passed by the Congress of the United States after each decennial census

Census

A census is the procedure of systematically acquiring and recording information about the members of a given population. It is a regularly occurring and official count of a particular population. The term is used mostly in connection with national population and housing censuses; other common...

 to determine the number of members which each state shall send to the United States House of Representatives

United States House of Representatives

The United States House of Representatives is one of the two Houses of the United States Congress, the bicameral legislature which also includes the Senate.The composition and powers of the House are established in Article One of the Constitution...

. 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 even larger than today's 435 members, the ratio, which is first settled by Congress before apportionment, has been raised after each census, as will be seen from the accompanying table.

Under	Census		Apportionment		Representatives
	Year	Population	Year	Ratio
Constitution	1787	n/a	1789	63,000	65
First Census United States Census, 1790 The United States Census of 1790 was the first census conducted in the United States. It recorded the population of the United States as of Census Day, August 2, 1790, as mandated by Article I, Section 2 of the United States Constitution and applicable laws...	1790	3,929,214	1793	33,000	105
Second Census United States Census, 1800 The United States Census of 1800 was the second Census conducted in the United States. It was conducted on August 4, 1800.It showed that 5,308,483 people were living in the United States of which 893,602 were slaves. The 1800 Census included the new District of Columbia...	1800	5,308,483	1803	33,000	141
Third Census United States Census, 1810 The United States Census of 1810 was the third Census conducted in the United States. It was conducted on August 6, 1810. It showed that 7,239,881 people were living in the United States of which 1,191,362 were slaves....	1810	7,239,881	1813	35,000	181
Fourth Census United States Census, 1820 The United States Census of 1820 was the fourth Census conducted in the United States. It was conducted on August 7, 1820 The total population was determined to be 9,638,453 of which 1,538,022 were slaves...	1820	9,633,822	1823	40,000	213
Fifth Census United States Census, 1830 The United States Census of 1830 was the fifth Census conducted in the United States. It was conducted on June 1, 1830. It determined the population of the 24 states to be 12,866,020 of which 2,009,043 were slaves. The center of population was about 170 miles west of Washington, D.C...	1830	12,866,020	1833	47,700	240
Sixth Census United States Census, 1840 The United States Census of 1840 was the sixth census of the United States. Conducted by the Bureau of the Census on June 1, 1840, it determined the resident population of the United States to be 17,069,453 — an increase of 32.7 percent over the 12,866,020 persons enumerated during the 1830...	1840	17,069,453	1843	70,680	223
Seventh Census United States Census, 1850 The United States Census of 1850 was the seventh census of the United States. Conducted by the Bureau of the Census on June 1, 1850, it determined the resident population of the United States to be 23,191,876 — an increase of 35.9 percent over the 17,069,453 persons enumerated during the 1840...	1850	23,191,876	1853	93,423	234
Eighth Census United States Census, 1860 The United States Census of 1860 was the eighth Census conducted in the United States. It determined the population of the United States to be 31,443,321 — an increase of 35.4 percent over the 23,191,875 persons enumerated during the 1850 Census...	1860	31,443,321	1863	127,381	241
Ninth Census United States Census, 1870 The United State Census of 1870 was the ninth United States Census. Conducted by the Census Bureau in June 1870, the 1870 Census was the first census to provide detailed information on the black population, only years after the culmination of the Civil War when slaves were granted freedom. The...	1870	38,558,371	1873	131,425	292
Tenth Census United States Census, 1880 The United States Census of 1880 was the tenth United States Census conducted by the Census Bureau during June 1880. It was the first time that women were permitted to be enumerators...	1880	50,155,783	1883	151,911	325
Eleventh Census United States Census, 1890 The Eleventh United States Census was taken June 2, 1890. The data was tabulated by machine for the first time. The data reported that the distribution of the population had resulted in the disappearance of the American frontier...	1890	62,622,250	1893	173,901	356
Twelfth Census United States Census, 1900 The Twelfth United States Census, conducted by the Census Office on June 1, 1900, determined the resident population of the United States to be 76,212,168, an increase of 21.0 percent over the 62,979,766 persons enumerated during the 1890 Census....	1900	75,568,686	1903	194,182	386

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

Reynolds v. Sims

Reynolds v. Sims, 377 U.S. 533 was a United States Supreme Court case that ruled that state legislature districts had to be roughly equal in population.-Facts:...

, the electoral district

Electoral district

An electoral district is a distinct territorial subdivision for holding a separate election for one or more seats in a legislative body...

s 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

General ticket

General ticket representation is a term used to describe a particular method of electing members of a multi-member state delegation to the United States House of Representatives...

and they are called "congressmen-at-large."