So: by popular demand (my mum) a quick guide.

The 25th amendment was ratified- i.e. became part of the US constitution in 1967. The amendment is basically about clarifying the presidential and vice presidential succession.

You see, Article 2 of the Constitution says "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"

But this is quite vague.

So when W.H. Harrison was the first president to die in office in 1841, it wasn't really clear what powers his VP, John Tyler, would have, nor what he would be called (president? acting president?)

Tyler took the oath and became president. This set a precedent.

Until the 1960s, convention dictated that if a president died in office, then their VP became president, was called the president, and had the powers of the president. Fine.

But this was not spelled out in law. And there were some outstanding questions:

1. If a VP becomes President, then how is the VP replaced?

2. What if the president is unwell, or unable to perform his role for some other reason? How can that be managed?

JFK's assassination in 1963 made Congress decide to actually sort this out- and this is what eventually became the 25th amendment.

It does a few things. First, it sets out in law that, yes, if a president dies (or resigns, or is removed from office), the VP becomes president.

Second, it says that if a VP succeeds to the presidency because of the death/resignation of the president, then the new president (the ex-VP) can appoint a new vice president, as long as Congress agrees.

Third, it sets out what happens if the president is unable to perform their duties-- and how that is determined. This is covered by Sections 3 and 4 of the 25th amendment- and that's what people have been talking about this week.

How does it work? Well, if the VP and a majority of the Cabinet OR a review body appointed by Congress deem that the president is "unable to discharge the powers and duties of his office", then the VP immediately becomes acting president.

BUT...

If the president disputes this- by writing a letter to Congress- then he becomes president again UNLESS the VP and majority of Cabinet send another letter (within 4 days) insisting that the president is still unable to fulfil their duties. If that happens...

Both houses of Congress have to meet within 48 hours and vote on the issue. 2/3 of the members of each House would have to agree that the president can't discharge their duties and that the VP should assume the presidency.

(Separately, the President can also write to Congress themselves to say they cannot temporarily fulfil their duties-e.g if they are going to undergo surgery or are unwell. The VP becomes acting president until the president tells Congress he's able to resume his duties.)

Has the 25th amendment been used? Well, the bit about appointing a new VP if the office of VP is vacant has been used. Nixon used it to appoint Ford as his VP after Agnew, his original VP, resigned.

Then when Nixon quit, Ford became President, and appointed Rockefeller his VP.

BUT section 4 of the 25th amendment- about removing a president from office if they are unfit to perform their duties- has only ever been used in fiction (as West Wing fans will know). Not in reality.