An American president is not chosen directly by the people but by a group of officials known as the electoral college, in a manner prescribed by the US constitution and a complex set of state and federal laws. In theory, the electoral college chooses the candidate who won the most votes - but not always.
How does the electoral college work?
Each state has a number of electors in the electoral college proportionate to its population: the sum of its number of senators (always two) and representatives in the House.
Technically, Americans on election day cast votes for electors, not the candidates themselves, although in most cases the electors' names are not on the ballot.
California, the most populous state, has 55 electoral votes. A few small states and the District of Columbia have only three.
Today, the electoral college has 538 electors, and in all but two states, Maine and Nebraska, all of the state's electors are awarded to the winner of the popular vote within that state.
A candidate needs to win 270 electoral votes - half of the total plus one - to win the White House.
Part of a presidential candidate's grand strategy entails drawing a map of states the candidate can and must win to gather 270 electoral votes.
What does all this US jargon mean?
Why was the system chosen?
When the United States was founded in the late 18th Century, a national campaign was virtually impossible given the size of the country and the difficulty of communication.
Also, the US at the time had little in the way of national identity, states were jealous of their rights, political parties were suspect, and the popular vote somewhat feared.
The framers of the constitution in 1787 rejected both the election of the president by Congress and election by direct popular vote, on the grounds that people would vote for their local candidate and the big states would dominate.
The Southern states favoured the electoral college system because while slaves had no votes, under the constitution they were tallied as three-fifths of a person in the census.
- 2000: Republican George W Bush wins the White House with 271 electoral votes, though Democrat Al Gore won 540,520 more votes
- 1888: Republican Benjamin Harrison elected president with 233 electoral votes, though Democrat Grover Cleveland won 100,456 more votes
- 1876: Republican Rutherford B Hayes wins with 185 electoral votes, even though Democrat Samuel J Tilden won 264,292 more votes
- 1824: After four candidates split the electoral college, the House elects John Quincy Adams even though Andrew Jackson won more popular votes and electoral votes
Isn't it unfair that the winning candidate might get fewer popular votes?
Indeed, this is seen as a major drawback of the system.
Since 1804, four presidents have been elected who did not win the popular vote.
Most recently, in 2000 Al Gore won 48.38% of votes nationwide compared to George Bush's 47.87%. Yet Mr Bush won because he got 271 electoral votes compared with 266 for Mr Gore.
The winning votes came from Florida, whose 25 votes all went to Mr Bush even though he won only 537 more popular votes.
Another drawback is that in many states the result is a foregone conclusion and there is thus little incentive for the individual to vote. It is also a disincentive for candidates to campaign there.
For example, large states California, Illinois and New York are solidly Democratic and Texas is solidly Republican.
So what are the advantages?
The electoral college system is respected for its historical roots and because it does usually reflect the popular vote (48 out of 52 elections since 1804 produced a popular mandate).
It also gives greater weight to smaller states - one of the checks and balances the US constitution values.
For example, the largest state, California, has 12.03% of the US population but its 55 electoral college votes represent only 10.22% of the college total.
Wyoming, a sparsely populated state, has 0.18% of the US population but its three seats in the electoral college give it 0.56% of the college votes.
The college system also means that a candidate needs to get a spread of votes from across the country.
What happens if no candidate gets a majority of electoral college votes?
Under the 12th amendment to the US constitution, the House of Representatives elects the president.
Each state delegation, however, has only one vote, which means that the majority party in each delegation controls the vote. An absolute majority of states is required for election.
The vice-president is chosen by the Senate, with senators having an individual vote.
This has happened only once since 1804, when the electoral college system took its current shape with the 12th amendment.
In 1824, four candidates split the electoral vote, denying any one of them a majority.
Democrat Andrew Jackson had the most electoral votes and the greatest share of popular votes and expected to be president.
But the fourth-place finisher, House Speaker Henry Clay, thought little of Jackson and persuaded the House to back second-place finisher John Quincy Adams. Adams was voted in as president.
Are the electors bound to vote for their candidates?
In some states they have a free vote but in practice they vote for the candidates they are pledged to, while in other states they are required to do so.
Only nine electoral votes have been cast against the state's instructions by so-called "faithless" electors, and no result has been changed by it, according to the Congressional Research Service. And in 2000 an elector from the District of Columbia abstained.
If the result is extremely close, a "faithless" elector could cause real trouble. The issue would probably have to be decided by the courts.
The electors are chosen by the parties before the election, often in a vote at a convention. The electors then meet in state capitals after the election to cast their votes. The results are formally declared to the Senate on 6 January. The new president is inaugurated on 20 January.