This story was published before the Iowa caucus, which was held on Feb 1, US time.
On Monday (Feb 1), residents of the US state of Iowa will cast the first votes in the months-long race for the White House.
Local meetings known as caucuses, are organised by the Republican and Democratic parties as they launch the process to determine who will be their presidential nominees in the election on Nov 8.
Twelve Republican candidates and three Democratic hopefuls are vying for the lead in the race.
Here's a quick look at how the process works and why the vote in Iowa is crucial:
Who gets to be a presidential nominee?
A series of votings - caucuses and primaries - are held in every state and overseas territory, starting in February, which determine who becomes each party's official presidential candidate.
Iowa state kicked off the voting with its caucus on Feb 1, and New Hampshire follows with its primary on Feb 9. The South Carolina primary and Nevada caucus are next. On March 1, known as "Super Tuesday", 14 states (and one territory) hold their primaries and caucuses all on the same day. By mid-June, all the states will have completed the process.
The winner of each primary or caucus collects a number of delegates - party members with the power to vote for that candidate at the national party conventions in July, where candidates are formally confirmed. The more state contests a candidate wins, the more delegates will be pledged to support him or her at the convention.
The Republican candidate will need 1,237 delegates to win a majority, while the Democratic contender must secure 2,383.
What's the difference between a primary and a caucus?
For voters, primaries and caucuses mean two very different experiences. A primary is an official election where voters cast their ballots for their preferred candidates in secret, as they would for any other election. But in caucuses, participants typically discuss the candidates in an open forum.
Some primaries and caucuses are "closed", meaning that only registered party members can take part, while some are "open" and any registered voter can participate.
How does the Iowa process work?
Americans who turn 18 years old by election day on Nov 8 are eligible to participate in the Iowa caucuses.
The Democrats will host some 1,681 caucuses and the Republicans roughly the same number.
Each party organises precinct meeting locations, mostly in public places like schools, libraries and other government buildings, as well as in private homes. The Republican and Democratic caucuses are often located close to one another, sometimes just down the hall in the same building.
There will also be a virtual "tele-caucus" for US military personnel deployed out of state or overseas, and "satellite caucuses" at locations including nursing homes, where people are not mobile.
For both parties, most meetings begin at 7pm local time (9am on Tuesday, Singapore time).
The precinct results will be delivered via a new digital application specially developed by Microsoft, which will replace an outdated telephone system.
How do the processes of the two parties differ?
Republican voters gather at the appointed time and after some organisational formalities, candidates' representatives each make a short speech urging voters for support.
A secret ballot is then held. The polling station reports the results to the party, which aggregates the results from the precincts and announces the winner who has received the most votes at the state level.
The process is a lot more complicated for the Democrats. There is no secret ballot and some critics argue that the process subverts the "one person, one vote" principle proclaimed by the US Supreme Court.
Following initial formalities, supporters of each candidate gather in one area of a caucus room - for example, backers of Mrs Hillary Clinton in one corner and those favouring Mr Bernie Sanders in another corner.
Candidate groups lacking a minimum of 15 per cent support are eliminated, and the supporters are then invited to join another candidate group. It is during this realignment that group leaders will try to rally supporters for their candidates and a new vote is taken.
The supporters for each candidate group are counted, and a candidate is attributed a certain number of delegates proportionally to the county conventions. Due to rounding, a stronger candidate may end up with the same number of delegates as one with fewer caucus supporters.
The party then calculates a ratio by which a candidate's delegates to the state convention are determined, based on the number of county delegates a candidate receives.
The candidate who accrues the most state delegates is proclaimed the winner of the caucus.
Why does Iowa matter?
By the data, Iowa is of small significance to the election. The state has just three million of the country's nearly 319 million residents and so it does not have many votes in the Electoral College that will eventually elect a president from the parties' nominees.
In the presidential nominations too, Iowa does not mean much in terms of sheer numbers: for both the Republicans and the Democrats, the state makes up about one per cent of delegates who will later choose the nominees.
But Iowa can mean everything because of the simple fact that its contest comes first. While a candidate does not need to win Iowa to win its party's nomination, results in Iowa are followed closely by the media and are an early signal of how the country as a whole will respond to the candidates.
It also sets the stage for the first primary eight days later on Feb 9 in New Hampshire. If a candidate does not place highly in the early states, support and campaign donations typically begin to dry up, which means that Iowa often is successful at winnowing the field.
In a race with many candidates, how a politician fares in Iowa can determine whether he or she will remain on the ballots for the rest of the US.
Source: Agence France-Presse, New York Times, US News, Washington Post
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