The 538 members of the US Electoral College meet Monday in the capital of each of the 50 states to designate a successor to Barack Obama as president of the United States.
A candidate must obtain an absolute majority of the vote – or 270 of the 538 – to claim the presidency.
The system originated with the US Constitution in 1787. It establishes the rules for indirect, single-round presidential elections by universal suffrage (not entirely universal: blacks and women could not vote at the time).
The country's Founding Fathers saw this as a compromise between direct presidential elections with universal suffrage, and an election by members of Congress – an approach rejected as insufficiently democratic.
Who are the electors?
There are 538 in all.
Most are local elected officials or local party leaders, but their names do not appear on ballots, and their identities are almost entirely unknown to the general public.
Each state has as many electors as it has members in the House of Representatives (a number dependent on the state's population) and in the Senate (two in every state, regardless of size).
Populous California, for example, has 55 electors; Texas has 38; and sparsely populated Vermont, Alaska, Wyoming and Delaware have only three each, as does the District of Columbia.
In the November 8 election, Donald Trump won a majority of electors (306 of the 538), but his Democratic rival Hillary Clinton won the popular vote by a margin of nearly three million.
While this situation is not unprecedented (see George W Bush's victory over Al Gore in 2000), this year's particularly nasty election campaign exacerbated the tensions and ill will over the outcome.
Millions of Americans who consider Donald Trump unfit to occupy the Oval Office have signed an online petition calling for Republican electors to block his election. Thirty-seven of them would have to do so to prevent the real estate mogul from being elected.
Nothing in the constitution or in federal law requires electors to vote one way or another.
Laws in some states do require electors to respect the popular vote (the so-called "faithless electors" who defy the popular vote generally face a simple fine), while other states impose no such requirement.
History shows that it is extremely rare for an elector to defy the expressed will of his or her state's voters. While a few have done so, they have never changed the final choice of the person to occupy the White House for a four-year term.
While Electoral College members vote on Monday, the states have until December 28 to transmit their "Certificates of Vote" to the Congress and the National Archives in Washington, which will then immediately post them online. The formal announcement of the name of the next president will be made by the Congress on January 6.