Q&A: US primaries and caucuses
The route to a spot on the ballot in November's US presidential election runs through a series of electoral contests known as primaries and caucuses.
This is the process by which supporters of the Democratic and Republican parties, in each US state, determine which candidate they would like to represent their party in the presidential poll.
How does it work?
The parties' presidential nominees are formally chosen at national conventions held in the late summer. There, delegates sent by each state party vote for the candidate chosen by the voters back home.
The voters have their say in primary elections or party caucuses held between early January and the summer.
The more state contests a candidate wins, the more delegates will be pledged to support the candidate at the national convention.
Why did Iowa and New Hampshire come first?
No particular reason, it just happened that way. This year Iowa holds its caucuses on 3 January and New Hampshire holds the first primary on 10 January.
Critics of the two states' role argue they do not merit the influence because they are not representative of the US population as a whole.
Iowa and New Hampshire are relatively small and rural and are overwhelmingly white.
But the states argue the voters are serious and politically-educated and put the candidates through their paces.
What is Super Tuesday?
Super Tuesday is a day when a lot of states hold primaries or caucuses simultaneously.
In 2000, 16 states held primaries on 7 March, at which about 60% of all delegates were up for grabs.
In 2004, Super Tuesday split in two. There was a Mini-Tuesday (or Super Tuesday I) on 3 February, followed by a Super Tuesday II on 2 March. California, Ohio and New York all held their votes on Super Tuesday II.
On 5 February 2008, 24 states took part in "Super-Duper Tuesday", including California, New York, Illinois and New Jersey.
Caucus or primary - what's the difference?
A primary is a traditional election, where a broad electorate of voters cast secret ballots at polls open all day. The winner of the primary election takes the state's delegates to the national convention.
A caucus is a meeting of registered party voters and activists that takes place at an appointed time of day or evening.
Caucus procedures vary according to state law, but in most states, such as Iowa, voters meet in private homes, schools and other public buildings to discuss the candidates and the issues.
They then hold a vote of the caucus to chose a candidate and elect delegates pledged to support that candidate at county conventions.
County convention delegates elect delegates in turn to state conventions, where delegates to the national conventions are chosen.
At Democratic caucuses, the voters sometimes publicly divide into groups, gathering in different corners of a room to show their support for the different candidates, and delegates are allocated accordingly.
Republican caucuses usually take the form of a secret ballot, the results of which inform the allocation of delegates.
Are the caucuses and primaries held at the same time every election year?
No, and the national parties are frequently at odds with their state affiliates over timing.
States often vie with one another to hold their contests earlier in the season in an effort to boost their influence, with Iowa and New Hampshire striving to protect their status as the first in the nation caucuses and primary.
From 1972 to 1992 the exercise began in late January (or occasionally early February) and the nominations were not usually settled until the first Tuesday in June, when California, New Jersey and Ohio held their primaries.
In 2008 they began on 3 January, and the Republican nomination was effectively settled on 5 February, the earliest ever Super Tuesday (though John McCain only became certain of victory on 4 March).
The Democrat battle between then-Senators Barack Obama and Hillary Clinton continued until June - more than five months.
Is a long nomination contest a bad thing?
Yes and no.
Spending several months criss-crossing the country meeting voters is a good test of a candidate's strengths and weaknesses and breadth of support ahead of the gruelling general election fight in the autumn.
The long nomination calendar gives the parties an opportunity to build lists of voters, supporters and potential campaign volunteers for the general election.
Also, a little-known candidate who does well in the early caucuses or primaries has a chance to attract financial support, and organise a strong campaign.
The shorter the primary campaign, the less time there is for an outsider to gather momentum.
However, a long primary season can mean the eventual nominee has less time to prepare for the general election battle against the other party's candidate.
It could leave the party divided, and exhaust donors needed for the general election.