<P> The ability of a president to appoint new justices depends on the occurrence of a vacancy on the Court . The Constitution provides that justices "shall hold their offices during good behavior" (unless appointed during a Senate recess). The term "good behavior" is well understood to mean Justices may serve for the remainder of their lives, although they can voluntarily resign or retire . A Justice can also be removed by Congressional impeachment and conviction . However, only one Justice has been impeached by the House (Samuel Chase, in 1805) and he was acquitted in the Senate . Moves to impeach sitting justices have occurred more recently (for example, William O. Douglas was the subject of hearings twice, once in 1953 and again in 1970), but they have not reached a vote in the House . No mechanism presently exists for removing a Justice who is permanently incapacitated by illness or injury, both unable to resign and unable to resume service . </P> <P> Because Justices have indefinite tenure, it is impossible to know when a vacancy will next occur . Sometimes vacancies arise in quick succession, as in the early 1970s when Lewis Franklin Powell, Jr. and William H. Rehnquist were nominated to replace Hugo Black and John Marshall Harlan II, who retired within a week of each other . Sometimes a great length of time passes between nominations such as the eleven years between Stephen Breyer's nomination in 1994 and the departures of Chief Justice Rehnquist and Justice Sandra Day O'Connor (by death and retirement, respectively) in 2005 and 2006 . </P> <P> It is also theoretically possible for Congress to create additional vacancies by expanding the Court itself . The United States Constitution does not specify the size of the Supreme Court, but in Article III it authorizes the Congress to fix the number of justices . The Judiciary Act of 1789 called for the appointment of six justices . As the country grew geographically, Congress increased the number of justices to correspond with the growing number of judicial circuits . In 1807, the court was expanded to seven members, allowing Thomas Jefferson to appoint Thomas Todd as a new Justice . The expansion to nine members in 1837 allowed Andrew Jackson to appoint John Catron, and Martin Van Buren to appoint John McKinley; and the addition of a tenth seat in 1863 allowed Abraham Lincoln to name Stephen Johnson Field to the Court . However, at the request of Chief Justice Salmon P. Chase, Congress passed the Judicial Circuits Act (1866) which provided that the next three justices to retire would not be replaced; thus, the size of the Court should have eventually reached seven by attrition . Consequently, one seat was removed in 1866 and a second in 1867 . However, this law did not play out to completion, for in the Judiciary Act of 1869, also known as the Circuit Judges Act, the number of justices was again set at nine, where it has since remained . The new seat created in 1869 was filled the following year by Ulysses S. Grant's appointment of Joseph P. Bradley - the last justice to be appointed to a newly created seat on the Court . </P> <P> President Franklin D. Roosevelt attempted to expand the Court in 1937, seeking to appoint an additional justice for each incumbent justice who reached the age of 70 years 6 months and refused retirement; under Roosevelt's proposal, such appointments would continue until the Court reached a maximum size of 15 justices . Ostensibly, the proposal was made to ease the burdens of the docket on the elderly judges, but the President's actual purpose was to pack the Court with justices who would support New Deal policies and legislation . This plan, usually called the "Court - packing Plan", failed in Congress and proved a political disaster for Roosevelt . The balance of the Court shifted with the retirement of Willis Van Devanter and the confirmation of Hugo Black in August 1937 . By the end of 1941, Roosevelt had appointed seven Supreme Court justices and elevated Harlan Fiske Stone to Chief Justice . </P>

Who determines the number of justices on the supreme court