A supporter listens to Republican presidential candidate, Ohio Gov. John Kasich speak at Baldwin Wallace University in Berea, Ohio, March 15, 2016. Jeff Swensen/Getty Images

When Ohio Gov. John Kasich pulled out a do-or-die win in his home state Tuesday night, preventing Donald Trump from picking up 66 delegates, he brought the Republican Party one step closer to a contested convention in Cleveland this summer.

Party insiders have been talking about the possibility of a contested convention for months, hoping they can nominate someone other than Trump if no candidate secures the 1,237 delegates required to win by the end of the primary season. Trump has since pulled ahead in the delegate count and holds a nearly 300-delegate lead over his competition after Tuesday’s primaries. But at the rate he’s been winning delegates, it is still very likely that the New York businessman will fall short of the threshold that he needs to win outright.

If Trump — or some other, even less likely candidate — cannot lock down a majority of the delegates, the GOP will begin its national convention in July without a presumptive nominee for the first time since 1976, when Ronald Reagan challenged Gerald Ford (and lost). This would lead to lots of backroom discussions and create a complex situation. But before we get there, here is a primer on what you need to understand about all the talk about contested and brokered conventions.

What is the difference between a contested and a brokered convention? A convention is contested if no candidate finishes the primary season with a majority of the delegates. For the Republicans this year, that number is 1,237. At this point, candidates can try to convince the small number of uncommitted delegates to support them on the first ballot. If a candidate gets a majority of delegates on that vote, he wins the nomination. This is what happened in 1976 when Reagan initially prevented Ford, the incumbent president, from winning a majority, but Ford defeated him on the first ballot. No convention of either party has gone to a second ballot since 1952.

If no one gets the necessary majority on the first ballot, the convention turns into a “brokered” event. Most states require their pledged delegates — those awarded in primaries and caucuses — to stick to those choices only for the first round. After that, most delegates can switch allegiances, and so campaigns will try to win a majority in subsequent rounds of voting.

Republican Winners of State Primaries and Caucuses | InsideGov

How likely is a contested or brokered convention to happen? The short answer is very. With so many Republican candidates running at first in this cycle — three remain after Tuesday — they have split up the vote in many of the primaries and caucuses so far, meaning no candidate is within striking distance of the magic delegate number. But Gregory Magarian, an election law professor at Washington University in St. Louis, said the Ohio primary results have made a contested convention even more likely.

“If Trump had won Ohio, because it’s winner-take-all, of course, he would have been in a commanding position to get a delegate majority,” Magarian said, adding that Trump’s loss is more significant than Kasich’s win since the Ohio governor is still far behind in the delegate count.

It is technically still possible for Trump to get the right number of delegates, Magarian said, but a contested convention is “substantially more likely” after Ohio.

“If this were any other candidate in any other year, the party has a strong interest in getting that person over the finish line because you want to avoid a brokered convention and you want to coalesce around a candidate as early as possible,” Magarian said. “But in this case all of those considerations are reversed or absent.”

What are the rules? In any of these situations, the rules are very complicated. We explained what happens during the first ballot above, but if a convention gets to a second or third round, many things can change.

House Speaker Paul Ryan, R-Wis., will lead the convention, and members of the Republican National Committee, including its chairman, Reince Priebus, will also oversee the logistics of the event.

Every state has its own rules about how delegates are chosen and how those delegates can vote at the convention. Some states will keep their delegates locked in to the state’s primary or caucus results no matter how many ballots are needed, while others release their delegates after the first round of voting and some fall in between. California, for example, does not release its delegates until a third vote.

Delegates are typically party members from each state, but they are not all in leadership positions and they are technically autonomous once unbound. However, Magarian said that the national and state parties, in addition to the candidates, will try to convince delegates to vote for certain candidates.

“I suspect that some state parties are going to be able to exercise more discipline than others. The deciding factors will be relevance and power,” Magarian said. “The state party leadership, particularly in larger delegations, are going to be trying to rally their troops and trying to get them to vote strategically. There’s going to be dealmaking between candidates, delegates and the national party if they still have an agenda, which is likely to be to stop Trump.”

Delegates Awarded to Republican Candidates | InsideGov

Can the rules change? Yes, and they are likely to change this year. Every four years, the party’s Rules Committee meets before the convention begins. It includes 112 members and consists of two representatives — one man and one woman — from each of the 56 states and territories that participate in the convention.

This means the selection of those delegates to the Rules Committee is very important because they can determine significant changes that could help or prevent a candidate from being nominated. In 2012, the pre-convention meeting was used to add a requirement known as Rule 40 that said candidates needed to win a majority of the delegates in at least eight states to be considered viable for the nomination.

Only Trump meets that standard right now. The committee could change that rule before the 2016 convention. It would need to be changed if people who have not been involved in the primaries, like 2012 nominee Mitt Romney, are being considered as possible alternative nominees.

What happens to delegates assigned to candidates who drop out of the race? Like other delegate rules, this also varies by state. Some states will force their delegates who have been pledged to candidates who are no longer running, such as Florida Sen. Marco Rubio or Dr. Ben Carson, to stick with those candidates during the first round of voting.

But other states will reallocate their delegates before the convention, ensuring that everyone is voting for a candidate still in the race.

“They’ll convene a meeting of the delegates and figure out how to recommit the state’s delegates through a process with the party,” Magarian said. “I would guess it’s a process driven by the original vote or by the state party leadership.”

What could a brokered convention mean for the Republican Party? The outcome of a brokered convention is difficult to predict, because if it happens, it is likely to come down to conversations and persuasion among party elites, delegates and the various candidates left standing.

Magarian said he believes the nominee from a brokered convention is most likely to be someone already being considered, whether that is Trump, Kasich, Texas Sen. Ted Cruz, or someone like Mitt Romney or Ryan, who some establishment party members would like to see as options.

“In some ways, if Kasich shows up with a few hundred delegates, you could argue that he is very unqualified because he went before the voters and they did not choose him,” Magarian said. “You might go with a Romney or a Ryan, because you got rid of the whole primary process, but at least you’re coming out clean.”

Both Romney and Ryan have so far said they are not interested in the nomination, but things could change between now and Cleveland.

But the larger cost of a brokered convention could spell trouble for the Republican Party. Trump’s supporters have been very loyal to him, and many are not traditional Republicans, so they are unlikely to be happy with any other option. New Jersey Gov. Chris Christie, who dropped out of the race and endorsed Trump, said Tuesday a brokered convention would be “dangerous.”

Trump himself has often pointed to how many first-time voters he is bringing out, and said Wednesday morning that people would be upset if he is not the nominee. “I think you would have riots,” he told CNN.

There has been lots of talk about a third-party option, and while that may still be a stretch, many Republicans have said they are worried that a brokered convention could make it difficult to unite the party in time to compete in November.