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Three days after closed, the United States and the world still do not have final results from the presidential election, although Democrat was on the brink of unseating .

The wait has fueled tension across the polarized nation, with Trump alleging without evidence that Democrats are engineering fraud.

But the delay was widely expected, often for reasons specific to individual states, which under the US system each conduct their own polls:

Competitive states take longer

California, the nation's most populous state, was quickly called for Biden after polls closed Tuesday. But such calls are in fact projections by news outlets rather than official results, meaning that it takes longer to get an accurate picture in narrowly divided states.

"The closer the races, the longer it takes," Kathy Boockvar, secretary of the vital state of Pennsylvania, explained to reporters.

States also have diverse deadlines on receiving absentee ballots, especially those coming from the military or other citizens living overseas.

North Carolina has held off on counting at least 171,000 ballots - which could make the difference - as by law it accepts votes that arrive by mail through November 12 so long as they were postmarked by Election Day.

Similarly, Nevada, which also has a tight race, will count ballots postmarked by Election Day so long as they arrive by November 10.

Also causing delays are provisional ballots, which are issued to voters if there is confusion about their registration and need verification.

Deluge of mail-in ballots

With concerns about the Covid-19 pandemic, states accustomed to limited numbers of absentee votes have been deluged with ballots mailed by citizens who do not want to risk voting in person.

Some 65.2 million of the record 160 million Americans who voted this year did so by mail, according to an estimate of the US Elections Project.

In Pennsylvania, the Republican-led legislature rejected an effort to let authorities count ahead of Election Day, contributing to the scenario in which its biggest city Philadelphia - a Democratic stronghold - reported returns last.

Some places have unique factors delaying vote-counting such as Chatham County in tightly fought Georgia, where an election division and a registration board separately look at ballots.

Legal challenges 

The Trump campaign has seized on the delay to demand a halt to counting in states where it is behind, notably Pennsylvania whose Republican Party approached the US Supreme Court.

Republicans had been fighting for months against allowing Pennsylvania to count ballots postmarked by Election Day if they arrive by Friday.

In Wisconsin, where Biden eked out a narrow victory in results made known by early Wednesday, the Supreme Court ruled that only votes received by Election Day would count.

Most states allow rival parties to observe counting but challenges on the rules have taken time, with Trump's supporters taking particular issue with Philadelphia's rule that watchers stay at least 15 feet (4.5 meters) away due to Covid risks.