Section 2 of the 25th Amendment
Text of Section 2:
"Whenever there is a vacancy in the office of the Vice President, the President shall nominate a Vice President who shall take office upon confirmation by a majority vote of both Houses of Congress."
The provisions of Section 2 to date have been implemented twice since its adoption: to appoint and install Gerald Ford as Vice President in 1973, and then again the following year to install Nelson Rockefeller as Ford's successor.
Author's Notes and Comments
Thanks to a remarkable series of events, during a 436 day period in 1973-74 the United States of America had three different Vice Presidents: Spiro Agnew, Gerald Ford, and Nelson Rockefeller, two of which rose to that office thanks to the provisions of Section 2 of the 25th Amendment.
When Spiro Agnew resigned from the Vice Presidency on October 10, 1973, Ford was nominated as his successor. Ten months later, Ford would ascend to the presidency upon the resignation of Richard Nixon, and would appoint Nelson Rockefeller as his successor.
Without the 25th Amendment, following the Agnew and Nixon resignations House Speaker Carl Albert would have acted as President from August 9, 1974 to January 20, 1977.