People have been talking about the possibility of a “constitutional crisis” since before President Trump’s election. And in the wake of Trump’s executive order restricting the ability of people from seven predominantly Muslim nations to travel to the U.S., we’re once again hearing that term a lot. Rep. Don Beyer, a Virginia Democrat, used the phrase to describe reports of Customs and Border Protection officials snubbing members of Congress and refusing to abide by a judge’s ruling delaying the enforcement of the order. And the Washington Post’s Aaron Blake investigated whether Trump’s disparaging remarks about a federal judge who ruled against the administration could be considered a constitutional crisis.

So what exactly is a constitutional crisis? We should be clear about what does — and, more importantly, does not — merit this description. It’s possible to have a major political crisis even if the Constitution is crystal clear on the remedy, or to have a constitutional crisis that doesn’t ruffle many feathers.

Political and legal observers generally divide constitutional crises into four categories:

1. The Constitution doesn’t say what to do.

The U.S. Constitution is brief and vague. (Compare it to a state constitution sometime.) This vagueness has one major advantage: It makes an 18th-century document flexible enough to effectively serve a 21st-century society. But sometimes the Constitution leaves us without sorely needed instructions, such as when William Henry Harrison became the first president to die in office in 1841. At the time, it wasn’t clear whether the vice president should fully assume the office or just safeguard the role until a new president could somehow be chosen. (It wasn’t until 1967 that the 25th amendment officially settled the question.) When Vice President John Tyler took over, no one was sure if he was the real president or merely the acting president, nor was anyone certain what should happen next. Tyler asserted that he was, in fact, the new president, and since then, vice presidents who have had to step into service as chief executive have been treated as fully legitimate, but early confusion took its toll on the perceived legitimacy of Tyler’s presidency.

In this way, informal precedents and practices have filled in many of the gaps in the Constitution’s text over time. Constitutional amendments have also clarified issues such as succession rules and term limits.

So what could cause this kind of constitutional crisis during Trump’s presidency? One area where the Constitution has never been clarified — much to the chagrin of advocates for a limited presidency — is the question of emergency powers. The Constitution is completely silent on what powers the president has to respond to crises, although some scholars argue that protecting the nation in a state of emergency is an inherent power of the chief executive. Plenty of presidents have interpreted it this way — Abraham Lincoln did when he suspended habeas corpus during the Civil War, for example, as did Harry Truman when he tried to seize the steel mills during the Korean War.

Trump, for his part, has talked a lot about terrorist attacks, even suggesting that the media might be covering them up. Given presidents’ history of using crises and threats to shore up their emergency powers, it’s easy to imagine that in the wake of a terrorist attack or war, Trump might enact policies that would lead Congress or the courts to challenge whether he had the authority to take those actions. That would be a constitutional crisis.

2. The Constitution’s meaning is in question.

Sometimes the Constitution’s attempt to address an issue is phrased in a way that could allow multiple interpretations, leaving experts disagreeing about what it means and making it difficult or impossible to address a pressing problem. In this way, both the Great Depression and the Civil War created constitutional crises. The problem sparked by the Civil War is obvious: The fight rested on a bunch of unsettled constitutional questions, the biggest of which was about slavery and the federal government’s ability to control it, a subject on which the Constitution was silent. And while the Constitution provided information on how a state could join the union, it didn’t say whether one could leave it or how it would go about doing so. It obviously took a war to resolve this crisis.

When it comes to the Great Depression, the issue is harder to see. By the time Franklin D. Roosevelt was elected president in 1932, there were calls for the president to declare martial law to more forcefully address the economic situation because, well, people were starving and homeless. Contemporary interpretations of the Constitution offered very limited views about how the federal government could intervene in the economy, limits FDR wasn’t inclined to heed. Indeed, the Supreme Court overturned many of the New Deal’s main provisions. In 1937, Roosevelt threatened to change the makeup of the court, asking Congress to add seats (which is within their power) so that he could pack the court with new appointees who were more likely to vote his way. (The Constitution is notably silent on the proper number of seats on the court — a fact Merrick Garland surely knows by now.) Roosevelt argued that the people had elected him twice and had installed a Democratic Congress, thus endorsing his new vision for economic regulation. Even though the court-packing plan did not violate any specific provision of the Constitution, many felt Roosevelt was abusing the Constitution’s vagueness to claim near-dictatorial powers, creating a crisis. Ultimately, several sitting justices chose to side with FDR’s economic plans, and the president backed off on his court-packing proposal.

Presidential politics frequently spur constitutional conflict when Congress feels the executive branch has claimed too much power. Congress censured Andrew Jackson for taking aggressive steps to destroy the Second Bank of the United States instead of waiting for its charter to expire. Clashes over war powers are common enough that Congress, overriding Richard Nixon’s veto, enacted the War Powers Resolution in 1973 in an effort to lay out a procedure for the president to notify Congress when American troops are sent into a conflict. But these clashes rarely rise to the level of a constitutional crisis, even though experts consistently warn about increasing, unchecked executive power.

Trump has only been in office for three weeks, so it’s hard to say how this type of constitutional crisis might come up during his presidency. So far, Trump has relied heavily on executive orders and hasn’t revealed much in the way of a legislative agenda. The main controversies of his administration thus far have been over the substance of his actions, and the immigration order was blocked mainly on the grounds that it violated the constitutional rights of individuals, not that it represented executive overreach. But if Trump goes too far in issuing executive orders that affect domestic politics, or if he takes heavy-handed action with the states, the overreach question could come up.

Another possible crisis that would fall into this category: impeachment. It doesn’t come up all that often, but one source of constitutional confusion is the impeachment clause in Article II, Section 4, which states, “The President, Vice President and all civil officers of the United States, shall be removed from office on impeachment for, and conviction of, treason, bribery, or other high crimes and misdemeanors.” What are “high crimes and misdemeanors?” The nation debated this during the Clinton impeachment but didn’t really get any closer to a definition. Since there’s little agreement about the conditions under which a president can be removed from office, we might be in for a constitutional crisis if the House attempts to initiate the impeachment process. Then again, maybe not. The criteria are murky, but the process for removal is clear: If the House drafts articles of impeachment and the Senate votes to convict, the president is out. If he refuses to leave, however, that would certainly spark a constitutional crisis.

3. The Constitution tells us what to do, but it’s not politically feasible.

This category of constitutional crisis can crop up when presidential elections produce contested and confusing results. In the 2000 presidential election, when George W. Bush and Al Gore were separated by just a few hundred votes in Florida, the tipping-point state whose electoral votes would determine the winner, the state’s election results remained contested for weeks due to a number of irregularities and a secretary of state who seemed determined to cut a recount short. In theory, the Constitution allowed for various solutions to this problem: Congress could have decided which of Florida’s electors to recognize, or Congress could have determined that neither candidate had achieved a majority in the Electoral College and let the House of Representatives decide on a president (per the process spelled out in the 12th Amendment). Such outcomes, while certainly constitutional, would have been politically infeasible, creating a significant legitimacy crisis for the new president.

The last time the House of Representatives chose a president was in 1824, and this decision was widely decried as corrupt and created a massive backlash against John Quincy Adams’ administration. In 2000, the Supreme Court stepped in instead, cutting the recount short. The court’s decision ended the dispute in Bush’s favor, and Gore gave a televised statement announcing that while he disagreed with the decision, he would accept it. In this case, the court arguably created a constitutional crisis by stepping in where it wasn’t technically needed, but Gore’s acceptance of the decision defused the situation. Had Gore refused to abide by the decision, the crisis could have become far more serious.

In a somewhat similar case, the results of the 1876 election were hotly disputed when Democrat Samuel Tilden won a majority of the popular vote but enough state delegations remained in dispute to deny him a majority of the Electoral College. Congress appointed a 15-member special commission to address the growing crisis. Members came up with a compromise under which Republican candidate Rutherford B. Hayes would become the president, but Union soldiers would be withdrawn from Southern states, effectively ending the post-Civil War Reconstruction period. This election remains possibly the most disputed presidential contest to date.

It’s fair to ask whether these sorts of crises are even constitutional in nature — just because we don’t want to follow the Constitution doesn’t mean there’s a constitutional crisis. But if following the Constitution is impractical, that is itself a form of crisis. Presumably, we won’t encounter an election-related constitutional crisis of this type until 2020.

However, other scenarios might create a similar dilemma. There has been a lot of talk about the 25th amendment, which allows other governing officials to remove the president and put the vice president in charge. Like most of the Constitution, there’s some room for interpretation, but this kind of removal is definitely established in the text. It’s possible to imagine a situation in which the Cabinet decides this is the best course of action but faces hurdles in selling it to the millions of voters who chose Trump in November, sparking a crisis.

4. Institutions themselves fail.

The Constitution’s system of checks and balances sets the various branches against each other for the laudable purpose of constraining tyranny. However, due to partisan polarization, individual corruption, or any number of other reasons, sometimes the political institutions in these arrangements fail, sending the governmental system into a crisis. This was the type of constitutional crisis commentators were seemingly referring to in describing reports that Customs and Border Protection agents (members of the executive branch) weren’t following orders from the judicial branch.

In theory, clashes between different parts of government could regularly produce constitutional crises, but in reality, they often don’t. Had Nixon ignored the Supreme Court ruling ordering him to turn over tapes of conversations he had recorded in the Oval Office, that would have been a huge crisis of this genre. But he didn’t.

Government shutdowns are a milder example. During the brief shutdowns in the Clinton and Barack Obama years, some government functions remained in place, and in both cases, agreements were eventually reached. But these situations illustrate how the Constitution doesn’t always provide safeguards or guidelines for making a decision when governing bodies reach a stalemate. The provisions of the Constitution set up political incentives for elected leaders to ensure that the government runs. When these don’t work, there’s not much recourse.

True constitutional crises are rare. The Constitution is set up so that power is shared between the president, Congress and the courts, and between the federal government and the states. This cuts down on vacuums where no one has clear authority, instead creating situations where multiple people or institutions are empowered to act. Serious constitutional crises occur when our institutions are rendered ineffective, which is usually about politics more than process, and often has less to do with how institutions were designed than with how legitimate they are perceived to be.

The last type of constitutional crisis — when different parts or branches of government are at loggerheads — might be the one we are most likely to see during Trump’s administration. If Trump continues to strain democratic norms and push political boundaries, Congress, the courts or even members of his own administration could push back. Those conflicts could be resolved deliberately and thoughtfully, with an eye toward what the founders would do. Or not.