United States presidential candidate Joe Biden’s projected win in Pennsylvania has put him over the top in the Electoral College, the threshold needed to win the White House, according to the Associated Press news agency.

The AP said on Saturday that Biden was projected to win the key battleground state – giving him 284 Electoral College votes. The AP soon after called a win for Biden in Nevada, putting him at 290.

Under the Electoral College system, which is defined in the US Constitution, 50 US states and the District of Columbia are apportioned electors based on their population.

A presidential candidate needs to win 270 electoral votes out of 538 to win the presidency, notwithstanding the popular vote.

President Donald Trump has not conceded and in a statement issued after the announcement said “this election is far from over”. His campaign has promised to challenge the results in various states.

“Beginning Monday, our campaign will start prosecuting our case in court to ensure election laws are fully upheld and the rightful winner is seated,” Trump said in the Saturday statement.

The pathways to 270 Electoral College votes described below are all projections released by The Associated Press, and they will need to be certified by state officials in the coming days and weeks.

Recounts are all but certain in several key states, including Georgia and Wisconsin, and Trump has said he will try to challenge the results in court.

Here is a look at how both Biden and Trump got their Electoral College votes.

Pennsylvania seals Biden’s victory:

Biden started with a baseline of 187 Electoral College votes from the District of Columbia and 14 solidly Democratic states, including two of the country’s largest: California, which is worth a whopping 55 Electoral College votes, and New York, which holds 29.

Biden easily picked up Colorado, Virginia and New Hampshire on election night, which added 26 votes to his column to bring him to 213.

As votes were tabulated and reported the next day, Biden then began beating Trump in key battleground states.

He secured Minnesota, Wisconsin and Michigan, adding 36 votes to bring his total to 249.

He also won parts of Nebraska and Maine, which have a split Electoral College system, giving him an additional four electoral votes between them – bringing his total to 253.

Crucially, Biden won the state of Arizona, which had gone to Trump in 2016, bringing him 11 votes and putting him within striking distance of victory, at 264 total votes.

Biden’s margin in Arizona was tight and while the Associated Press and Fox News had called it for the former US vice president, other major media organisations held off as incoming mail ballots appeared to favour Trump.

That is when the process appeared to stall on Thursday, as the count of millions of mail ballots in all-important Pennsylvania and Georgia ground on with the presidency hanging in the balance and no clear winner.

Then, on Saturday, Biden’s win in Pennsylvania put him over the top in the Electoral College with 284 votes. The AP soon after called a win for Biden in Nevada, putting him at 290.

The Republican leader started with only 122 votes from 19 solidly Republican states.

Texas had briefly appeared close, but Trump won its 38 electoral votes to bring his early count to 160.

Trump won some, but not all of the battleground states he had won in the 2016 elections.

Ohio, Iowa, and Florida went to the president, adding 53 votes and getting him to 213 in the Electoral College.

He also won a congressional district in Maine, bringing him an additional vote – and bumping his total up to 214.

But Trump fell short in the all-important Midwest, losing Michigan and Wisconsin to Biden.

In the end, Trump needed Pennsylvania just to stay in the race.

Even then he would have still needed a long-shot win in Nevada and a reversal of the outcome in Arizona, two southwest states that went to Biden.