BIDEN versus Trump: the lineup in the United States election this year has long been a foregone conclusion, with a rematch between the two presidents appearing all but certain.
But what would happen if, for any number of reasons, one of the two contenders does not end up taking part in the November vote?
Apart from immense political confusion, here is what would happen, according to the rules and experts:
What would cause President Joe Biden, an 81-year-old Democrat, or ex-president Donald Trump, the 77-year-old Republican, not to be on the ballot?
Age, for one. Either candidate would enter office in January 2025 as the oldest US president ever.
Although neither has reported any serious health issues, statistics suggest that both face heightened risk of mortality or serious medical incident because of their advancing years.
Voluntary withdrawal? "It's a fabulously ridiculous fantasy," said Rachel Bitecofer, a Democratic political strategist.
Even if Republicans wanted to change their rules and switch candidates, following a Trump legal conviction, "they still wouldn't, because it'd cause a total meltdown in their base".
Biden says he is the best qualified candidate, despite polls that indicate his age is off-putting to voters.
"What are you supposed to say — 'Oh, he's fine, he's going to run a triathlon tomorrow'? I mean, he's 81 years old," said Democratic congressman Adam Smith.
Still, "nobody of note decided to run against him, so here we are".
Trump meanwhile insists he will run despite a possible criminal conviction before the election that theoretically could see him face decades in prison.
To designate a party's formal nominee, delegates from each state attend their party's summer nominating convention to officially anoint a candidate based on primary voting.
If Biden or Trump were to exit the race before the end of the primaries, final say would go to the delegates at the convention.
And those delegates, Elaine Kamarck of the Brookings Institution said in a recent note, are "8,567 people you've never heard of", ordinary Americans who happen to be politically active.
This has not happened since president Lyndon B. Johnson made the shock announcement on March 31, 1968, in the middle of the Vietnam War, that he would not seek reelection.
Since then, conventions have been well-oiled affairs, whose outcomes have been known in advance since they are determined by the primaries.
But this year, the departure of a candidate could result in "a convention where the result may not be known ahead of time ... the kind of no-holds-barred event that nominating conventions held between 1831 and 1968", said Kamarck.
And what if something happens to one of the candidates after being nominated at the convention?
One of the parties' governing bodies, the Democratic National Committee or the Republican National Committee (RNC), would nominate a new candidate.
On the Republican side, the RNC is undergoing a reshuffle and Trump has suggested placing his daughter-in-law Lara in a leadership role, which would give his camp enormous influence in choosing a replacement.
A strong, but not automatic, candidate to take Biden's place would be his vice-president, Kamala Harris. Otherwise, a number of strong Democratic politicians — Governors Gavin Newsom of California, Gretchen Whitmer of Michigan and Josh Shapiro of Pennsylvania are mentioned — might be called on.
But Trump has not yet announced his choice for VP.
And as Hans Noel, professor of government at Georgetown University, said, Trump had belittled his party's other heavy hitters throughout the primaries.
Meanwhile, could a strong third-party candidate emerge? So far, no independent candidate is posing any danger to America's dominant two-party system.
In 1992, Texas billionaire Ross Perot, running as an independent, managed to win nearly 19 per cent of the popular vote.
But in the end, because of the vagaries of the American electoral system, he did not receive a single one of the votes that matter most: those of the 538 members of the Electoral College that ultimately decide the winner.
The writers are from Agence France-Presse