The 25th Amendment to the U.S. Constitution deals with presidential authority in the event of death or removal from office, and was ratified in 1967, in the wake of John F Kennedy's assassination.
What does the 25th Amendment say?
It is in four sections, all dealing with the president leaving office during his or her elected term.
The first section states that the vice president takes over the Oval Office if the president dies or resigns – or is removed – something which the original Constitution did not clearly state.
Presidents of course can be removed by impeachment, a feature of the constitution from the start. They can also be removed through the 25th Amendment - of which more below.
Section II states that if the vice president dies, or resigns – or is fired – both the House and Senate have to confirm a new vice president. Until 1967, presidents could change vice presidents mid-term on their own if they got the vice president to agree to resign - not something that actually happened, but which was possible in principle.
Section III makes clear that the a president can temporarily delegate his powers to the vice president, and later reclaim them when he - or she - is capable of serving. This is most often invoked if a president is under the influence of surgical anesthetic for a short period of time.
Section IV is the amendment's most controversial part: it describes how the president can be removed from office if he is incapacitated and does not leave on his own.
The vice president and 'a majority of either the principal officers of the executive departments or of such other body as Congress may by law provide' must write to both the president pro tempore of the Senate and the Speaker of the House, saying that 'the President is unable to discharge the powers and duties of his office.'
The term principal officers of the executive departments would normally would mean the cabinet secretaries.
So at least eight of the president's 15 most senior Cabinet members, who , together with the vice president, must agree that a president should be removed before any plan can move forward.
Notifying the House Speaker and the Senate president pro tempore is the act that immediately elevates the vice president to an 'acting president' role.
The deposed president can contest the claim, giving the leaders of the bloodless coup four days to re-assert their claims to the House and Senate.
Congress then has two days to convene – unless it is already in session – and another 21 days to vote on whether the president is incapable of serving. A two-thirds majority in both houses is required to make that determination.
As soon as there is a vote with a two-thirds majority, the president loses his powers and is removed, and the vice president stops acting and is sworn in as president.
But if 21 days of debate and votes ends without a two-thirds majority, the president gets back his powers.
What could happen to trigger the 25th Amendment?
Vice President Mike Pence and eight of the 15 'principal' Cabinet members would have to agree to notify Congress that President Donald Trump was incapable of running the country.
That group is made up of the Secretary of State, Treasury Secretary Steve Mnuchin, Secretary of Defense , Attorney General, Interior Secretary, Agriculture Secretary, Commerce Secretary, Labor Secretary, Health and Human Services Secretary, Transportation Secretary, Energy Secretary , Education Secretary, Veterans Affairs Secretary and Homeland Security Secretary.
Their formal notification would go to the House Speaker and, in the senate, to the 'president pro tempore', the Senate's most
