Footnotes

1. Constitutional Apportionment (Article 1, Section 2).
2. No change was made after the 14th Census (1920), as Congress could not agree on a method for apportionment.
3. As new states joined the Union, the House assigned a specific number of Representatives to that state. These Representatives are not represented in the table until after the next census and subsequent apportionment. See footnote 4 for exceptions.
4. Congress either anticipated the admission of the following states to the Union or they were admitted in the interim period between the census and the apportionment: Vermont (1791), Kentucky (1792), Missouri (1821), California (1850), Kansas (1861), New Mexico (1912), and Arizona (1912). Representatives apportioned to these states are included in the table for the previous census.
5. Congress included representation for California from figures in 7th Census (1850), as it was scheduled to become a state that year; however, when the apportionment law passed in 1852, Californias census returns remained incomplete. Congress made a special provision, allowing California two representatives, as prescribed in the act of admission, until a new census and apportionment was made after 1860. However, Congress allotted one more seat to California in the 37th Congress (1861–1863), bringing the state total to three.
6. Formerly part of Massachusetts, when Maine achieved statehood in 1820, Congress assigned the new state one At-Large Representative, leaving Massachusetts with its allotted 20 Representatives. In the 17th Congress (1821–1823), the final Congress before the apportionment following the 4th Census (1820), Congress reassigned seven Massachusetts Representatives to Maine, leaving Massachusetts with 13 Members of the House.
7. The 8th Census (1860) allotted 11 Representatives to Virginia. In 1863, when West Virginia formed from a part of Virginia which had refused to secede from the Union, the new state was assigned three Members of the House. Upon Virginias return to the Union in 1870, eight Members represented it.

Sources:

Congressional Directory, various editions
Martis, Kenneth.  The Historical Atlas of United States Congressional Districts, 1789–1983 (New York: MacMillan Publishing Company, 1985): 4–6.
Silbey, Joel, editor, Encyclopedia of the American Legislative System, volumes 1–3 (New York: Charles Scribner’s Sons, 1994).
U.S. Census Bureau, “Congressional Apportionment” (accessed 23 April 2007).