The night of Tuesday 3 November was always going to be memorable, with those who love Donald Trump and those who hate him glued to their television sets.
But it has now become apparent that this will really be an election unlike any ever before - because we will not know the results for some time.
Why might there be delays to the election result announcement?
The 2016 election was called for Trump at about 02:30 EST (07:30 GMT) after victory in Wisconsin put him over the 270 electoral votes needed. This year will be very different.
An unprecedented number of Americans - 97.1 million by Monday, well over twice the number from 2016 - have voted by mail, given coronavirus concerns, and some states will count ballots that are delivered after the election if they are postmarked by a deadline.
In Washington state, for example, mail-in ballots can still be counted on 23 November. In Alaska state officials don’t even begin counting mail-in ballots until around 10 November.
Those two states are relatively unimportant for the final result in the presidential race, in that Washington is strongly Democrat and Alaska is fairly solidly Republican.
However, swing states will also face challenges reporting quickly.
In Pennsylvania — key for both Mr Biden and Mr Trump - mail-in ballots can be counted three days after polls close, following a battle that made it all the way to the Supreme Court. On Thursday all eyes were indeed on the state, which had counted 89 per cent by around 12am EST.
The governor of Pennsylvania, Tom Wolf, said there were still millions of ballots to be counted on Wednesday morning.
Donald Trump’s campaign on Wednesday declared victory in the state, and also filed a suit in the Supreme Court asking for the remaining ballots not to be counted.
The campaign said that it was suing to temporarily stop the vote count in Pennsylvania, citing a lack of “transparency” in the count process.
Justin Clark, Trump's deputy campaign manager, said the campaign is "suing to stop Democrat election officials from hiding the ballot counting and processing from our Republican poll observers."
He said the campaign wants "to temporarily halt counting until there is meaningful transparency and Republicans can ensure all counting is done above board and by the law."
Read more: The electoral college system, explained
What have Trump and Biden said?
Both candidates were well aware of the fact that there might be delays.
Joe Biden was urged by Hillary Clinton not to concede on election night "under any circumstances because I think this is going to drag out" - he has said that he will only accept “the full results”.
On election night, at around 1am, Mr Biden told his supporters to “keep the faith” and urged them to “be patient” as the counting continued.
He spent the evening watching the returns come in from his home in Wilmington, Delaware, then drove downtown by motorcade to make his statement outside the Chase Center.
He told a gathering of supporters that his hopes for victory remain high despite the uncertainty and cautioned them that it could take a day or longer to know who won.
He told them: “Your patience is commendable.”
On Wednesday Mr Biden urged the counting of all votes.
"Here, the people rule. Power can't be taken or asserted. It flows from the people," he said.
"And it's their will that determines who will be president of the United States and their will alone.
“And now after a long night of counting, it's clear that we are winning enough states to reach 270 electoral votes needed to win the presidency.”
Donald Trump took it a step further, saying the process was “a major fraud on our nation.”
He cast the night as a disenfranchisement of his voters.
He said: “We will win this and as far as I’m concerned we already have won it.
“We’ll be going to the US Supreme Court — we want all voting to stop.” In fact, there is no more voting — just counting.
Mr Trump had repeatedly stated that he might not accept the results at all. He is doing his utmost to warn of voter fraud and election rigging - something which experts say is extremely rare. The Brennan Center for Justice said rates for voter fraud in elections it had studied ranged from 0.0003 per cent to 0.0025 per cent.
“We have to prepare for the very strong probability that an election unlike any other we’ve ever had might take a little longer to accurately count with integrity,” said David Becker, executive director and founder of the Washington DC-based non-partisan Center for Election Innovation & Research.
“More time being taken to report results is not an indication of a problem.”
When will the election be over?
Pennsylvania, Nevada, North Carolina and Georgia are among the key states yet to declare their results. Arizona was declared a win for Mr Biden by both AP and Fox News, but the lead has diminished as more votes come in – a warning sign the state could still be in play.
Nevada is expected later on Thursday, and Pennsylvania perhaps even later.
That means we could have a declared winner by Thursday mid-morning at the earliest.
The primaries gave us some indication.
By 17 March, almost half of all states, 24, had held primaries. Five of those states heavily used vote-by-mail, as they had in previous elections. They reported their results in an average of 6.4 days, according to data analysed by the Washington Post.
The other 19 had mainly in-person voting, and reported results on average after 15 hours.
After 17 March, when the pandemic forced significant adjustments to voting systems, there were primaries in 23 more states, which took an average of four days to report nearly complete results. Nearly all of these states had traditionally relied on in-person voting.
New York processed 94 per cent of its votes within a few hours of polls closing. But counting the final six per cent of votes took more than 10 days, and one race in particular - for the congressional seat held by long-term Congresswoman Carolyn Maloney - took six weeks to be resolved.
That suggested from the start that it could take several days for the results to be clear, and, if both candidates accept the result, the matter will be solved.
What happens if Trump or Biden don’t accept the election result?
If either candidate is still not satisfied, a recount could be demanded.
In 2018, in Florida, a recount was demanded for three statewide races — for senator, governor and agriculture commissioner.
The machine recount was ordered on 10 November, with a deadline set for 8pm on 15 November.
That was still inconclusive, in the senate race, so Florida ordered its first ever recount by hand.
Read more: The US 2020 election results live
That was completed on 18 November, and officially approved on 20 November - two weeks to the day after the 6 November vote.
But even a recount result may be unlikely to resolve the issue: Mr Trump has said he believes the election result could end up in the Supreme Court.
Mail-In Ballot fraud found in many elections. People are just now seeing how bad, dishonest and slow it is. Election results could be delayed for months. No more big election night answers? 1% not even counted in 2016. Ridiculous! Just a formula for RIGGING an Election....
— Donald J. Trump (@realDonaldTrump) July 10, 2020
What would the Supreme Court involvement be?
This has happened before. In the 2000 election, Al Gore, the Democratic candidate, argued that his narrow margin of defeat in Florida should lead to a recount.
It took 36 days and the Supreme Court to decide against a recount to settle the contest in favour of Republican George W Bush. He, like Trump, had lost the popular vote.
Various legal challenges could emerge this year - everything from the identification requirements for postal voting, to the legality of Covid-related changes to voting.
Is there a “hard deadline” to all this?
Inauguration Day - always 20 January, or 21 January if 20 January falls on a Sunday - is set in the Constitution, under the 20th Amendment, enacted in 1933.
Before then, the “lame duck” period stretched on until March, and caused plenty of problems.
In the "secession winter" after the election of Abraham Lincoln, President James Buchanan sat back and watched as secessionists seized federal forts and arsenals. By March 1861, when Lincoln finally took office, the Civil War was nearly lost before it even began.
And in the midst of the Great Depression, the lame duck period of president Herbert Hoover seemed far too long. In response, the 20th Amendment was proposed and adopted.
So 20 January, 2021, is therefore considered the ultimate hard deadline.
And if there are still problems?
If the election result is not confirmed and accepted by then, the United States will be plunged into a deeply partisan constitutional crisis.