The 2024 GOP presidential primary is in full swing in Iowa this week, as candidates squeeze in final campaign stops ahead of the state's caucuses on Jan. 15.

Why it matters: While former President Trump has dominated polling, many will be watching how his main rivals, Florida Gov. Ron DeSantis and former UN Ambassador Nikki Haley, perform.

• Iowa's Republican caucus will be the star of this year's show, due to no significant challengers to President Biden on the Democratic side.

Below is our guide to the history of the Iowa caucuses and how they work.

When and where do the caucuses take place?

On Jan. 15, Iowa Republican caucus-goers will gather in locations like schools and churches to cast ballots in what will be the first test of the 2024 Republican presidential field.

• Starting around 7pm Monday local time, caucus-goers will show up at their local precincts. They'll listen to speeches, mingle and cast private ballots, similar to a straw poll.

Between the lines: The Democratic National Committee removed Iowa from its early contests last year at President Biden's request.

• Iowa Democrats have pivoted to a mail-in process. Cards must be postmarked and returned by March 5.

So, what is the importance of the Iowa caucuses?

Iowa's Republican caucuses will be straightforward.

• The in-person gatherings are meant to encourage conversations between neighbors, rather than just casting ballots.
• Most ballots are a blank sheet of paper where a voter will write the name of their preferred candidate.

Yes, but: While Iowa Democrats' system used to involve choosing a candidate by physically gathering in parts of a room, that's now a thing of the past.

• The change came after the 2020 Democratic caucuses' result-tallying glitches, which left no clear winner for weeks.
• This year, Democratic mail-in results won't be revealed until Super Tuesday, which is March 5.

The bottom line: Expect this year's results to trickle in more smoothly than 2020, due to Republicans' simpler process.

• Axios.com will have the latest tallies in real time.

How to participate in the caucuses

• You must be registered to vote in Iowa.
• You must be at least 18 years old on Election Day, Nov. 5.
• Your party affiliation on your registration must match the caucus you plan to participate in.
• Iowans can find their GOP precincts and Democratic precincts here. (Democrats will just be meeting to discuss party business.)

How the Iowa caucuses started

Iowa's first-in-the-nation status began as a fluke.

Flashback: In the 1960s, amid Vietnam War and anti-establishment protests, some in Iowa felt that everyday people were not being heard by political leaders, according to Iowa PBS.

• As a result, Iowa Democrats revamped their caucus system to create a more inclusive process.
• Presidential nominations would start at the precinct level and end at the state level, rather than the existing top-down system, to prioritize opinions at the grassroots level.

Yes, but: To make the new system work, the party needed to print new materials for thousands of precincts.

• And with an archaic mimeograph machine, that was going to take some time.

As a result, they set the caucuses earlier than all the other states in 1972.

• By 1976, Iowa Republicans scheduled their own caucuses for the same night.