2 U.S.C. § 2a

(a) On the first day, or within one week thereafter, of the first regular session of the Eighty-second Congress and of each fifth Congress thereafter, the President shall transmit to the Congress a statement showing the whole number of persons in each State, excluding Indians not taxed, as ascertained under the seventeenth and each subsequent decennial census of the population, and the number of Representatives to which each State would be entitled under an apportionment of the then existing number of Representatives by the method known as the method of equal proportions, no State to receive less than one Member. (b) Each State shall be entitled, in the Eighty-third Congress and in each Congress thereafter until the taking effect of a reapportionment under this section or subsequent statute, to the number of Representatives shown in the statement required by subsection (a) of this section, no State to receive less than one Member. It shall be the duty of the Clerk of the House of Representatives, within fifteen calendar days after the receipt of such statement, to send to the executive of each State a certificate of the number of Representatives to which such State is entitled under this section. In case of a vacancy in the office of Clerk, or of his absence or inability to discharge this duty, then such duty shall devolve upon the Sergeant at Arms of the House of Representatives. (c) Until a State is redistricted in the manner provided by the law thereof after any apportionment, the Representatives to which such State is entitled under such apportionment shall be elected in the following manner: (1) If there is no change in the number of Representatives, they shall be elected from the districts then prescribed by the law of such State, and if any of them are elected from the State at large they shall continue to be so elected; (2) if there is an increase in the number of Representatives, such additional Representative or Representatives shall be elected from the State at large and the other Representatives from the districts then prescribed by the law of such State; (3) if there is a decrease in the number of Representatives but the number of districts in such State is equal to such decreased number of Representatives, they shall be elected from the districts then prescribed by the law of such State; (4) if there is a decrease in the number of Representatives but the number of districts in such State is less than such number of Representatives, the number of Representatives by which such number of districts is exceeded shall be elected from the State at large and the other Representatives from the districts then prescribed by the law of such State; or (5) if there is a decrease in the number of Representatives and the number of districts in such State exceeds such decreased number of Representatives, they shall be elected from the State at large.