US:Across Iowa tonight tens of thousands of voters will venture out into sub-zero temperatures to participate in one of the world's most extraordinary but influential democratic exercises.
At more than 1,700 venues ranging from schools and courthouses to private homes, Democrats and Republicans will hold caucuses to choose presidential candidates.
The word caucus comes from a Native American term for a meeting of tribal chiefs and Iowans like to think of their caucuses as meetings of neighbours.
Few neighbourly meetings have such complex rules, however, requiring participants to stand around in groups for up to an hour, publicly advertising their political preferences.
Any US citizen or green card holder who lives in Iowa and who will be 18 by election day on November 4th can participate in the primaries and voters can register on the night to affiliate with the party of their choice.
The caucuses do not actually choose presidential candidates but rather delegates to county conventions which in turn choose Iowa's delegates to the party national conventions later this year.
The Republican caucus is a relatively straightforward straw poll and, unlike at its Democratic counterpart, the vote is by secret ballot with delegates awarded proportionately to each candidate.
At the Democratic caucus, supporters of each candidate have a chance to make their case and then participants form into groups in support of each candidate. Groups that fall below a specified threshold - usually 15 per cent - have to dissolve and their members realign with other groups until only three candidates remain. Competition for the second preferences of supporters of minor candidates can lead to all kinds of horse-trading, including promises to perform neighbourly chores such as shovelling snow out of the coveted voter's driveway.
Iowa's Democrats apportion delegates to each county according to the level of support shown there for the last Democratic candidates for president and governor, while counties divide their delegates among precincts on a similar formula. This means that heavily Democratic precincts have a disproportionate influence.
All the campaigns are mounting massive operations to bring their supporters to the caucuses, providing transport, babysitters and in some cases, holding pre-caucus parties with food and music. Most campaigns have booked venues in Des Moines for post-caucus events, where they can watch the results from 9pm before taking the earliest available flight to New Hampshire.