Every four years, Republicans and Democrats hold national conventions to choose their nominees for president of the United States. Since the 1970s, the process has played out in predictable ways. Going into the summer convention season, there’s usually a clear winner of state primaries and caucuses, making the official nomination process largely ceremonial.

But that wasn’t always the case. The history of U.S. presidential elections is filled with conventions that were anything but predictable. Before the mid-20th century, it often took multiple rounds of hotly contested voting for a party to pick its nominee. Those were known as “brokered” conventions, because the winner was ultimately determined by backroom deals struck by political power brokers.

“Brokered conventions used to happen a lot,” says Andrew Rudalevige, a professor of government at Bowdoin College. “In the pre-primary era, delegates weren’t pledged to a candidate going into the convention. You’d have the first ballot where everybody voted for their favorite candidate and then you’d really get down to business. It was pretty common for there to be a lot of horse trading on the convention floor.”

The Founding Fathers were opposed to the formation of political parties, and there are no rules or instructions in the Constitution for how to choose or nominate candidates for president. Instead, political parties and national party conventions were created in response to the system laid out in the Constitution for electing the president—the electoral college.

Article II, Section 1 explains that the president is chosen by electors from each state (equal to their total number of senators and representatives in Congress). To become president, a candidate must win a majority of all electoral votes. If no candidate wins more than 50 percent of the electoral vote, the decision goes to the House of Representatives.

Political parties formed almost immediately after George Washington left office, but it was often the case that multiple members of the same party ran for president. By splitting the vote, none of them won a majority, sending the election to the House.

That’s what happened in 1824, when four Democratic-Republicans ran for president, including John Quincy Adams, Henry Clay and Andrew Jackson. Jackson received the most electoral votes, but not enough to reach a majority. When the decision went to the House, Clay—who was Speaker of the House—convinced his allies to rally around Adams, denying Jackson the presidency.

Jackson was furious and won a rematch against Adams in 1828, but Jackson’s policies proved so divisive that his enemies formed a new anti-Jackson political party called the National Republicans. It was the National Republicans who held the first major national political convention in 1831 and nominated Clay.

Prior to this, presidential candidates were nominated through a series of party caucuses in state legislatures and Congress. With improved communication in the early 19th century, political news and information traveled faster. Party officials organized national conventions to rally nationwide support behind a single nominee for president.

Jackson’s supporters punched back against Clay by organizing the first Democratic National Convention in 1832. The modern Republican party, which formed in opposition to the spread of slavery in the western territories, held its first national convention in 1856.

The nomination process at national party conventions is similar to the electoral college. Each state is allocated a certain number of delegates. The candidate who receives a majority of votes out of all the delegates wins the party’s nomination. In the 2024 election, Democrats need at least 1,968 delegates to secure the nomination. Republicans need at least 1,215 delegates to win their party’s nomination.

The Democratic and Republican parties are free to make up their own rules when it comes to the process for choosing a presidential nominee. Over time, those rules have changed considerably, particularly when it comes to the allocation of delegates.

Before 1972, delegates to both the Democratic and Republican national conventions were free to pick whichever candidate they preferred. A lot of states held primaries and caucuses, but the results of those primary elections weren’t “binding.” Delegates to both party conventions didn’t have to go with the presidential candidate who won their state’s primary.

That changed after the disastrous 1968 Democratic National Convention, where anti-war demonstrators protested the nomination of Hubert Humphrey, the establishment candidate. Humphrey didn’t participate in any state primaries, but he still won the nomination, which infuriated progressive voters.

After Humphrey lost badly to Richard Nixon, the Democratic National Committee changed its convention rules in 1972. Democratic delegates were now bound by the results of their state primary or caucus. Each Democratic candidate is now awarded a set number of delegates based on the percentage of the state primary vote that they won. The Republicans followed suit and changed their convention rules so that most of their delegates are also bound or “pledged” to state primary election results.

Today, since almost all convention delegates are tied to the winner of their state’s primary, there isn’t much mystery as to how delegates will vote at each party’s national convention. If a candidate wins a majority of the primary vote, they are all but guaranteed to secure their party’s nomination.

In the past, though, there was no such guarantee. Many national party conventions were deeply divided, and it required multiple rounds of voting—and plenty of passionate debate and political horse trading—before delegates settled on a nominee.

Of the 60 national conventions (30 Democratic and 30 Republican) held between 1868 and 1984, 18 of them qualified as brokered conventions (10 Democratic and eight Republican), according to Pew Research. A brokered convention is any convention that requires more than one round of ballots to choose a nominee.

“You have factions within the parties,” says Rudalevige. “They were trading delegates for political favors and trying to build coalitions. It was not uncommon for voting to go into multiple ballots.”

The record-holder was the 1924 Democratic National Convention, which dragged on for 16 days. Back then, Democrats required a two-thirds vote to secure the nomination, not just a simple majority. With the vote evenly split between two fierce political enemies, Democratic delegates voted a whopping 103 times before reaching a consensus. In the end, neither of the frontrunners won. The nomination went to an unknown “dark horse” candidate named John W. Davis.

The last brokered convention happened more than 70 years ago. Coming into the 1952 Democratic National Convention, Senator Estes Kefauver had won 12 state primaries and was the clear frontrunner. But party officials were angry that Kefauver’s televised hearings about organized crime had implicated some prominent Democrats. So they threw their support behind write-in candidate Adlai Stevenson, who won the nomination after three rounds of ballots.

Even though it’s been decades since a party nomination was even seriously contested—Jimmy Carter and Edward Kennedy came close in 1980—it’s still possible to have a brokered convention. That’s because the rules tying delegates to primaries are thrown out if voting goes beyond the first ballot.

At the Democratic National Convention, if no candidate wins a majority of the delegates in the first round of voting, all delegates are “unbound,” meaning they can now vote for anyone they want, just like the old days. According to DNC rules, “Eligible delegates may vote for the candidate of their choice whether or not the name of such candidate was placed in nomination.” The only qualification is that the candidate is a registered Democrat.

There is one additional twist for Democrats: If ballots go into a second round, then so-called “Superdelegates” are allowed to vote, too. Superdelegates include Democratic party officials, elected officeholders like members of Congress and governors, and even former presidents. Superdelegates were created to bring some order to the 1984 convention after the mess between Carter and Kennedy in 1980. Superdelegates used to vote in the first round of ballots, too, but that changed in 2016.

The Republican National Committee has the same rule for its national convention. Roughly 7 percent of Republican delegates are “unpledged” to begin with, but if voting goes to a second round, then all delegates are freed from loyalty to the results of their state primaries.

Rudalevige notes that even in the first round of ballots, it’s not technically “illegal” for a delegate to vote against his or her “pledged” candidate, but it’s probably political suicide.

“Everybody who goes to these party conventions is a pretty loyal party member,” says Rudalevige. “They’re active within the party. They want a future within the party. Going back on their word would be a pretty good way to ruin that future.”