The route to running in November's US presidential election is via a series of elections known as primaries and caucuses.
This is the process by which supporters of the Democratic or Republican parties, in each US state, say which candidate they would like to see representing their party in the presidential poll.
How does it work?
Each state gets to send a certain number of delegates to the parties' national conventions in the late summer, where each party's nominee is formally chosen - and the primaries and caucuses determine which candidate those delegates vote for.
Broadly speaking, the more voters from a given state give their backing to candidate X, the more delegates pledged to support candidate X will be sent from that state to the national convention.
Why did Iowa and New Hampshire come first?
No particular reason, it just happened that way. This year Iowa held its caucuses on 3 January and New Hampshire held the first primary on 8 January.
It's sometimes argued that they don't deserve it, because they are not representative of the US as a whole - they are small (especially New Hampshire), they don't have big cities, and their populations are not very diverse.
The counter-argument is that both states have politically-educated voters who put would-be candidates through a lot of tough questioning.
What was Super Tuesday?
A day (a Tuesday!) when a lot of states hold primaries or caucuses simultaneously.
The practice began in the 1980s.
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?
Caucus procedures vary according to state law.
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 elect delegates to the 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.
In primary elections, all registered voters in a state directly vote for their preferred candidate.
Are the caucuses and primaries held at the same time every election year?
No. This year everything happened earlier.
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.
This year 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 looks set to continue until June, and with early start the contest will have lasted longer than ever - more than five months.
Is a long nomination contest a bad thing?
To spend several months criss-crossing the country meeting voters is a good test of a candidate's strengths and weaknesses, and it gives Americans lots of opportunities to decide which politician they like best.
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 - and that their own party is more divided.
Why did the primary calendar change?
Some states felt the established system gave too much prominence to the states which began the selection process, and too little to those at the end of the process, so in the 1990s a trend began for states to move their primaries and caucuses forward.
This just increased the disadvantage of coming last and gave states more of an incentive to rush to the front of the queue.
States such as Iowa and New Hampshire, which have traditionally come first, responded by moving their own events earlier still.
Did the Democratic and Republican parties attempt to stop states changing the electoral calendar?
The Democratic Party punished Michigan and Florida by refusing to accept their delegates at the national convention.
The Republican Party penalised Florida, Michigan, New Hampshire, South Carolina and Wyoming, by reducing their quota of delegates by half.
Do these penalties matter?
They could, in theory, if it's a close race.
The Democrats have closed the door to about 8% of delegates, the Republicans to about 5%.
It's possible the decisions to impose the penalties could be reversed before the convention. However, proposals by Democrats in Florida and Michigan to stage their contests again have foundered.