process. The Framers of the Constitution chose good behavior tenure to limit the power to remove justices and to ensure judicial independence. No constitutional mechanism exists for removing a justice who is permanently incapacitated by illness or injury, but unable (or unwilling) to resign. The only justice ever to be impeached was Samuel Chase, in 1804. The House of Representatives adopted eight articles of impeachment against him; however, he was acquitted by the Senate, and remained in office until his death in 1811. No subsequent effort to impeach a sitting justice has progressed beyond referral to the Judiciary Committee. (For example, William O. Douglas was the subject of hearings twice, in 1953 and again in 1970; and Abe Fortas resigned while hearings were being organized in 1969.) Because justices have indefinite tenure, timing of vacancies can be unpredictable. Sometimes they arise in quick succession, as in September 1971, when Hugo Black and John Marshall Harlan II left within days of each other, the shortest period of time between vacancies in the court's history. Sometimes a great length of time passes between vacancies, such as the 11-year span, from 1994 to 2005, from the retirement of Harry Blackmun to the death of William Rehnquist, which was the second longest timespan between vacancies in the court's history. On average a new justice joins the court about every two years. Despite the variability, all but four presidents have been able to appoint at least one justice. William Henry Harrison died a month after taking office, although his successor (John Tyler) made an appointment during that presidential term. Likewise, Zachary Taylor died 16 months after taking office, but his successor (Millard Fillmore) also made a Supreme Court nomination before the end of that term. Andrew Johnson, who became president after the assassination of Abraham Lincoln, was denied the opportunity to appoint a justice by a reduction in the size of the court. Jimmy Carter is the only person elected president to have left office after at least one full term without having the opportunity to appoint a justice. Presidents James Monroe, Franklin D. Roosevelt, and George W. Bush each served a full term without an opportunity to appoint a justice, but made appointments during their subsequent terms in office. No president who has served more than one full term has gone without at least one opportunity to make an appointment. Size of the court