WASHINGTON—Americans will vote for president on Nov. 3. But before then, there’s a nine-month, cross-county marathon for candidates that formally kicks off on Monday. That’s when the people of Iowa gather to caucus, beginning the nominee selection process that will decide who is on the general election ballot.

The U.S. “primary season” is a process unlike anything seen in Canada, or most of the rest of the world. Before the general election campaign formally begins, the two major parties hold a series of contests in every state and territory to determine their nominees. Most states hold “primaries” — one-party-only elections with votes conducted similar to those in a general election. Some states — including Iowa, Nevada and Maine — hold caucuses, in which voters gather in gymnasiums or meeting rooms for several hours and physically line up in support of their candidates, and then rearrange themselves as those candidates with the least support drop off the ballot.

To add to what can already be a confusing system, some states hold a combination of primaries and caucuses.

In all cases, the state contests elect delegates pledged to particular candidates, who then go to the national conventions of the political parties, where the nominee whose name will appear on the general election ballot is formally chosen. Mirroring the electoral college system that decides the general election, states send delegates to the convention based roughly on their population. Some states pledge all of their delegates to whoever wins their primary or caucus; others apportion delegates according to the percentage of the vote candidates receive.

Some election years, both parties run competitive contests through the primaries to choose a candidate. This year, no major candidates have emerged to challenge President Donald Trump for the Republican nomination, and even though two fringe candidates are running against him, some states have cancelled their Republican primaries to give all of their support to Trump. Twelve candidates remain in the race for the Democratic nomination, including polling frontrunners Joe Biden, Bernie Sanders, Elizabeth Warren, Pete Buttigieg, Michael Bloomberg and Amy Klobuchar.

Not all of the primary and caucus contests are equal: those at the beginning of the process hold considerable influence on how the rest of the season plays out, and can shape who is able to compete and who is not.

By tradition — and continuing this year — the Iowa caucuses are held first, at the beginning of February, followed by the first primary, in New Hampshire, just over a week later. Those two tiny, mostly white states pride themselves on narrowing the field and setting the terms of the contest for the entire country. Candidates have spent the better part of a year or more campaigning in those two states, behaving almost like they would in a campaign for mayor or city council. They meet voters at potluck dinners in their living rooms, go door-to-door, visit every bar and coffee shop they can. Voters in those states have come to expect to meet candidates personally before making their decisions, and the field typically narrows as candidates who don’t perform well drop off the ballot.

This year, it is thought that Warren, Buttigieg and Klobuchar need strong performances in Iowa and New Hampshire to be viable beyond those states.

Following soon after Iowa and New Hampshire in February are the caucus in Nevada and the primary in South Carolina, two larger and more ethnically diverse states.

And then, at the very beginning of March, is “Super Tuesday,” when 14 states and Democrats living abroad all vote on the same day — including in the giant and delegate-rich states of California and Texas.

Billionaire candidate Michael Bloomberg is bucking tradition by skipping the four early states this year, opening his campaign in the Super Tuesday states, where he hopes the races will still be undecided enough that he can scoop up a large number of delegates and join or surpass the frontrunners.

One week after Super Tuesday, six states hold their primaries, including the vote-rich battleground state of Michigan, and a week after that four more vote, including large swing states Florida and Ohio.

The primaries wrap up when the U.S. Virgin Islands territory votes on June 6. In most years, the candidate is essentially chosen well before that final contest, often by Super Tuesday or soon after, when a clear front-runner has emerged.

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That may be the case this year, though current polling suggests this could continue to be a competitive race until late in the primary season. It could even result in a “brokered convention” in July in which delegates and party members gather in Milwaukee without any candidate having a majority of the pledged delegates.

In that case, those in attendance would select the nominee in a series of run-off votes. If such an uncertain convention were to convene this year, it would be the first time for the Democratic party since 1984, when Walter Mondale began the convention 40 pledged delegates shy of a majority but won the nomination on the first vote due to the support of party elders called “superdelegates.” The last time either party held a convention where it took more than one ballot to determine the nominee was 1952, when the Democrats nominated Adlai Stevenson on the third ballot.