The Supreme Court of the United States is the highest ranking judicial body in the United States. Its membership, as set by the Judiciary Act of 1869, consists of the Chief Justice of the United States and eight associate justices, any six of whom would constitute a quorum. Article II, Section 2, Clause 2 of the Constitution grants plenary power to the President of the United States to nominate, and with the advice and consent (confirmation) of the United States Senate, appoint justices to the Supreme Court. Justices have life tenure, and receive a salary which is set at $255,500 per year for the chief justice and at $244,400 per year for each associate justice as of 2014.
The Supreme Court was created by Article III of the United States Constitution, which stipulates that the "judicial power of the United States, shall be vested in one Supreme Court," and was organized by the 1st United States Congress. Through the Judiciary Act of 1789, Congress specified the Court's original and appellate jurisdiction, created thirteen judicial districts, and fixed the number of justices at six (one chief justice and five associate justices).
Since 1789, Congress has occasionally altered the size of the Supreme Court, historically in response to the country's own expansion in size. An 1801 act would have decreased the Court's size to five members upon its next vacancy. However, an 1802 act negated the effects of the 1801 act upon the Court before any such vacancy occurred, maintaining the Court's size at six members. Later legislation increased its size to seven members in 1807, to nine in 1837, and to ten in 1863. An 1866 act was to have reduced the Court's size from ten members to seven upon its next three vacancies, and two vacancies did occur during this period. However, before a third vacancy occurred, the Judiciary Act of 1869 intervened, restoring the Court's size to nine members, where it has remained since.
While the justices of the Supreme Court are appointed for life, many have retired or resigned. Beginning in the early 20th century, many justices who left the Court voluntarily did so by retiring from the Court without leaving the federal judiciary altogether. A retired justice, according to the United States Code, is no longer a member of the Supreme Court, but remains eligible to serve by designation as a judge of a U.S. Court of Appeals or District Court, and many retired justices have served in these capacities. Historically, the average length of service on the Court has been less than 15 years. However, since 1970 the average length of service has increased to about 26 years.