The Constitution does not specify the size of the Supreme Court's membership. Consequently, administrative and political considerations have determined the varied statutory number of positions on that bench. The Judiciary Act of 1789 linked the court to organization of the lower federal courts. It created three circuits wherein circuit courts in each would be held by two Supreme Court justices and the resident district judge, thereby necessitating a Supreme Court of six. Although circuit courts required only a single justice beginning in 1793, court size remained at six until Congress in 1801 severed the justices from the circuit courts and pared the Court membership to five. Jeffersonian antipathy to the 1801 measure caused its repeal in 1802 and restoration of the circuit‐linked court of six (see Judiciary Acts of 1801 and 1802).
The number of justices thereafter increased when Congress, acknowledging the nation's westward expansion and the court's caseload exigencies, established additional circuits accompanied by requisite Supreme Court positions: the seventh in 1807; the eighth and ninth in 1837; the tenth on the Pacific Coast in 1863 (see Judiciary Act of 1837).
Politics influenced the increase to ten justices to secure a majority favorable to President Abraham Lincoln's war policies. But illnesses and associated absenteeism together with vacancies created by deaths resulted in the actual attendance of ten justices on only five occasions before antagonism toward President Andrew Johnson's Reconstruction program combined with dissatisfaction over the unwieldy number of members led to an 1866 statute reducing court size by attrition to seven. This measure effectively deprived the president of vacancies to fill (see Judiciary Act of 1866). Congress in the Judiciary Act of 1869 fixed the membership to nine, the number of circuits authorized by the 1866 act, creating a vacancy on the existing court then composed of eight justices. The number of authorized positions has remained at nine notwithstanding the expansionistic potential of President Franklin D. Roosevelt's failed court‐packing plan in 1937. Nevertheless, the number of justices may temporarily fall below nine because of unfilled vacancies due to deaths or retirements and of virtual vacancies due to the disability of sitting justices.
— Peter G. Fish
