Tonightthe nominating process for the U.S. presidential elections officially begins when voters in Iowa meet for the caucuses. Here are five factsyou should know about what has, since 1972, been the first electoral event of each election season:

1. A caucus is a meeting of supporters or members of a specific political party or movement. To participate in the Iowa Caucus, political supporters show up at a one of the 1,681 precincts (church, school munity center, etc.) at a specific time (Monday, February 1, starting at 7 p.m. CST).

2. For the Republicans, the process is straightforward: Caucus goers cast a ballot for their preferred candidate, and national convention delegates are awarded proportionally based on the results. For the Democrats, the process is plicated. As Rebecca Kaplan explains:

First, voters show up to their precinct site. Then they’ll divide into presidential preference groups for the candidates they are supporting. For a candidate to be awarded any delegates out of that precinct, they’ll need to be “viable”—that is, they must have the support of at least 15 percent (or, in some cases, more) of the people in attendance.

If a candidate is not viable, their supporters can try to win over other caucus goers to meet the required threshold. Or they can disband and support the viable candidates. Their other option is to remain mitted entirely.

Based on the final results of the preference vote, each candidate will receive a proportional number of the county convention delegates, and “state delegate equivalents.” The exact delegate selection continues at the county and state conventions later in 2016, but generally reflects the presidential preference vote.

3. In 2012, evangelicals participated in the Republican Caucus at twice the rate of their population in Iowa (28 percent of the state’s population identifies as evangelical yet prise 57 percent of Republican caucus participants. That year 32 percent supported Rick Santorum. Ron Paul received 18 percent of the evangelical vote while Newt Gingrich, Rick Perry, and Mitt Romney took 14 percent each. (No similar religious polling was conducted for the Democratic Caucus in either 2008 or 2012.)

4. On the Democratic side, in the last 10caucuses(1976 to 2012):

• 5 caucuses included an incumbent President or Vice-President (1980: Carter; 1984: Mondale; 1996: Clinton; 2000: Al Gore; 2012: Obama). The incumbent always won thecaucusand the nomination, and 3 of the 5 times they also won in the general election (1980: Carter; 1996: Clinton; 2012: Obama).

• Of the 5 times when there was no incumbent, only 2 won both thecaucusand the nomination (2004: Kerry; 2008: Obama). The non-incumbent caucuses predicted the Democratic nominee only 40 percent of the time and the eventual president only 20 percent of the time.

• In 1972 and 1976, mitted” received more support than the eventual nominee (1972: McGovern; 1976: Carter).

5.On the Republican side, in the last 10caucuses(1976 to 2012):

• 5 caucuses included an incumbent President or Vice-President (1976: Ford; 1984: Reagan; 1988: George H.W. Bush; 1992: George H.W. Bush; 2004: George W. Bush). The incumbent lost thecaucusand won the nomination only once (1988: George H.W. Bush).

• Of the 5 with no incumbent, only 2 won both thecaucusand the nomination (1996: Bob Dole; 2000: George W. Bush). The non-incumbent caucuses predicted the Republican nominee only 40 percent of the time and the eventual president only 20 percent of the time.