Super Tuesday is the name given to the day in an election year when a group of US states hold simultaneous contests to help decide the Democratic and Republican parties' presidential nominations.
The outcome of 5 February could seal the fate of some candidates
This year more states are participating than ever before, on 5 February, earning it the nickname Super-Duper Tuesday.
How many states are taking part?
Twenty-four states are holding nominating contests, plus American Samoa.
In three states, only the Democratic party is involved, and in two, only the Republican party.
SUPER TUESDAY STATES
Both parties: Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Georgia, Illinois, Massachusetts, Minnesota, Missouri, New Jersey, New York, North Dakota, Oklahoma, Tennessee, Utah,
Democrats only: Idaho, Kansas, New Mexico
Republicans only: Montana, West Virginia
In the other 19, which together host nearly half the US population, both parties are in action.
Democrats abroad are participating in their own Democratic caucus, with the help of internet voting.
Who are the front-runners?
There are two candidates, Hillary Clinton and Barack Obama, whose fight for the Democratic nomination appears too close to call.
Opinion polls put John McCain ahead of Mitt Romney in the race for the Republican nomination, with Mike Huckabee a distant third.
Will one Democrat and one Republican emerge as "a winner"?
Candidates compete in state primaries and caucuses because, if they are successful, the state will send delegates to the national party convention with a mandate to support their bid for the nomination.
It is at the convention that the nominee is officially chosen.
The better a candidate does in the primaries and caucuses, the more of these "pledged delegates" he or she will accumulate.
While a dead heat is possible, it is more likely that one candidate in each party will pull ahead of the other - either by a little, or by a lot.
Could a candidate actually clinch his or her party's nomination on Super Tuesday?
This often happens.
Democrat John Kerry did it in 2004, and in 2000 George W Bush and Al Gore both saw off their main rivals - John McCain and Bill Bradley - who ended their presidential bids two days later.
Mathematically speaking, a Democratic candidate needs the support of 2,025 delegates at the party convention in order to secure the nomination, and a Republican needs 1,191.
New York, Missouri, New Jersey, Arizona, Utah, Connecticut, Montana, Delaware, West Virginia
Modified winner-takes-all system used in California, Georgia, Alabama and Oklahoma
None of the candidates will reach these "magic numbers" on 5 February. But in theory they could establish such a lead that their rivals have no chance of stopping them.
In practice, though, on the Democratic side, election rules make this very unlikely - especially with two relatively evenly matched candidates.
On the Republican side - where nine of the Super Tuesday states have a winner-takes-all rule for allocation of delegates - it is easier for one candidate to establish a big lead. John McCain says he thinks "there's a very good chance that it'll be over on Tuesday".
If it's not over on Super Tuesday, when will it be over?
The next big date in the primary calendar is 4 March, when Texas (America's second most populous state) and Ohio both hold their primaries. Other milestones after that will be the Pennsylvania primary in April, and the North Carolina and Indiana primaries in May.
In theory, the primary season could end with the nominations still hanging in the balance. This is because, as well as the pledged delegates (allocated in accordance with the result of primaries and caucuses) both parties have a certain number of unpledged delegates, who are free to choose which candidate to back, and they may not make their choice until the convention.
This would turn the party convention into a competitive event, rather than a coronation, as it usually is.
The last contested convention was the Democratic convention of 1952, which (after Harry Truman's early withdrawal from the race) was the last time neither an incumbent president or vice-president seriously competed for the presidency.
Are any of the candidates' home states taking part in Super Tuesday?
Yes, all the main candidates' home states are taking part - Arizona (John McCain), Arkansas (Mike Huckabee), Illinois (Barack Obama), Massachusetts (Mitt Romney), and New York (Hillary Clinton).
Who can vote on Super Tuesday?
In some states, voters have to be registered with the Democratic or Republican parties to take part in the primaries. But more than half of the Super Tuesday states allow independents to vote. In some cases these voters register with the party on the day; in other cases there is no need to register at all.
In California and New Jersey, and some other states, these unaffiliated voters number in their millions, and could have a significant effect on the outcome.
When did Super Tuesday come into existence?
The term began to be used in the early 1980s when three southern states began holding simultaneous contests on the second Tuesday in March. But the first big Super Tuesday took place on 8 March 1988 (when George Bush senior effectively wrapped up the Republican nomination).