(CNN) -- Here is some background information about the line of succession for the president and vice president of the United States.
Presidential Succession: Article 2, Section 1, Clause 6, Constitution of the United States: In Case of the Removal of the President from Office, or of his Death, Resignation, or Inability to discharge the Powers and Duties of the said Office, the Same shall devolve on the Vice President, and the Congress may by Law provide for the Case of Removal, Death, Resignation or Inability, both of the President and Vice President declaring what Officer shall then act as President, and such Officer shall act accordingly, until the Disability be removed, or a President shall be elected.
In the case that the president can no longer serve, the vice president would serve as president.
If the vice president cannot serve, the line of succession falls to the speaker of the House, then to the Senate president pro tempore, then to Cabinet members.
The Cabinet line of succession is: 1. Secretary of State 2. Secretary of the Treasury 3. Secretary of Defense 4. Attorney General 5. Secretary of the Interior 6. Secretary of Agriculture 7. Secretary of Commerce 8. Secretary of Labor 9. Secretary of Health and Human Services 10. Secretary of Housing and Urban Development 11. Secretary of Transportation 12. Secretary of Energy 13. Secretary of Education 14. Secretary of Veterans Affairs 15. Secretary of Homeland Security
The 25th Amendment allows the vice president to serve as acting president temporarily in the case that the president is ill or otherwise temporarily unable to fulfill his or her official duties.
Eight vice presidents have assumed the presidency upon the death of the president; one upon the president's resignation.
The president pro tempore serves as leader of the Senate in the vice president's absence and is usually the senior member of the majority party.
The 20th Amendment provides that vice president-elect becomes president if the president-elect dies before his term starts but after the Electoral College has met.
No Cabinet member may become president unless he or she is over 35 years old and a U.S. citizen.
History: 1792 - The Presidential Succession Act passes, making the Senate president pro tempore next in line after the vice president to succeed the president.
1886 - Congress changes the law to put cabinet officers next in line after the vice president. Proponents of the act thought cabinet officers had better experience to serve as president.
1947 - President Harry S Truman signs the Presidential Succession Act of 1947, changing the line of succession to vice president, then speaker of the House, then Senate president pro tempore.
2003 - Legislation is introduced in the House to reform the line of succession and move the secretary of homeland security from last to eighth behind the attorney general.
2004 - Testimony before House Judiciary Committee by Rep. Brad Sherman (D-CA) explores reforming the presidential line of succession by removing both the speaker of the House and the president pro tempore of the Senate, and expanding the list to include the ambassadors to the United Nations and those ambassadors to the four permanent member nations of the United Nations Security Council.
March 9, 2006 - The USA Patriot Improvement and Reauthorization Act of 2005 is signed into law, adding the secretary of homeland security to the line of succession.
Vice Presidential Succession: Section 2 of the 25th Amendment of the U.S. Constitution: Whenever there is 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.
If the vice president is unable to serve, the president nominates a replacement, which must be confirmed by Congress.
If something happened to the president before a vice presidential nominee is confirmed, the line of succession of president would fall to the speaker of the House.
There is no timeline on when the president must nominate a replacement.
Before the 25th Amendment, there was no procedure for selecting a replacement for the vice president. The assassination of President John F. Kennedy spurred the movement for the 25th Amendment.
History: Seven vice presidents have died in office: George Clinton who served under both Thomas Jefferson and James Madison; Elbridge Gerry who served under Madison; William R. King who served under Franklin Pierce; Henry Wilson who served under Ulysses Grant; Thomas Hendricks who served under Grover Cleveland; Garret Hobart who served under William McKinley; and James Sherman who served under William Howard Taft
When Harry Truman succeeded Franklin D. Roosevelt as president in 1945, the vice presidential office remained vacant for four years.
After the assassination of John F. Kennedy, there was no vice president from November 22, 1963 until Hubert Humphries' inauguration January 20, 1965.
July 6, 1965 - The 25th Amendment is sent to the states and is ratified on Feb. 10, 1967.
1973 - Gerald Ford becomes the first vice president chosen under the 25th Amendment after Vice President Spiro T. Agnew resigns.
1974 - After Richard Nixon resigns and Ford assumes the presidency, Ford nominates Nelson Rockefeller to be vice president.