On Wednesday evening, President Donald Trump became the third President in American history to be impeached. The House of Representatives voted in favor of two articles of impeachment — accusations of abusing the power of his office and obstructing Congress’s investigation into his relationship with Ukraine — that Democrats argue fall under the umbrella of “high crimes and misdemeanors.”

Now that the House has impeached the President, the issue goes to the Senate, who will vote on whether to convict and remove the President from office in 2020.

According to the U.S. Constitution, a President can be impeached for committing “Treason, Bribery, or other high Crimes and Misdemeanors.” Treason and bribery are relatively clear, but what exactly are “high crimes and misdemeanors”? The answer, it turns out, is complicated, and has been evolving for hundreds of years.

Here’s what to know about the history of high crimes and misdemeanors.

What are high crimes and misdemeanors?

The phrase “high crimes and misdemeanors” appears in Article II section 4 of the U.S. Constitution:

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.

While he was in Congress, before becoming President through a different series of unusual Constitutional processes, Gerald Ford offered a famously cheeky explication of that sentence: “An impeachable offense is whatever a majority of the House of Representatives considers it to be at a given moment in history.” But most legal scholars disagree.

“I think that’s a little glib,” says Tom Ginsburg, a law professor at the University of Chicago Law School. “There’s an element of it which its true, but there is some content to the standard. Congress isn’t just making things up. There’s a consistent set of precedents now.”

There are currently two major legal disputes over the definition of “high crimes and misdemeanors.” The first is whether or not something in that category actually has to be a crime. Frank Bowman, a law professor at the University of Missouri School of Law and the author of High Crimes & Misdemeanors: A History of Impeachment for the Age of Trump, tells TIME he believes it doesn’t. “The defenders of the impeached officer always argue, always, that a crime is required,” he says. “And every time that misconception has to be knocked down again.”

He offers this example: “Let’s say the President were to wake up tomorrow morning and says, ‘All this impeachment stuff is kind of getting on my nerves. I think I’m going to go to Barbados for six months. Don’t call me, I’ll call you,’ and just cuts off all contact and refuses to do his duty,” Bowman theorizes. “That’s not a crime. It’s not violating a law. But could we impeach him? Of course we could — otherwise what’s the remedy? We have a country without a President.”

The second legal dispute is over whether all crimes are impeachable: If a President broke a law but it didn’t relate to his or her office, can that person still be impeached? This question came up prominently during the impeachment of President Bill Clinton, whom a judge determined to have lied under oath.

To understand the answer to both questions, constitutional scholars recommend we look to history.

Members of the House Judiciary Committee discuss articles of impeachment against US President Bill Clinton Dec. 11, 1998, on Capitol Hill in Washington, D.C. Paul Richards—AFP via Getty Images

What’s the Constitutional history of the term?

The concept of impeachment was used by the British Parliament as early as 1376, as a legislative safeguard against overreach by the aristocracy, and the terms in question were part of the process early on.

“In England a lot of the impeachment cases had relied on this language of ‘high crimes and misdemeanors’ from the 1640s onward,” Bernadette Meyler, a law professor at Stanford Law School, explains.

But the phrase didn’t have a set definition in British practice; it was used to describe whatever thing the person was being impeached for, according to Bowman. There were several things for which people were impeached during this era: ordinary crimes, treason, corruption, abuse of power, ordinary incompetence and misbehavior in relation to foreign policy. Notably, the King could not be impeached.

When the framers of the U.S. Constitution realized they needed a way to remove executive officials who abused the nature of their positions, they decided to add a definition for an impeachable offense. Though many suggestions were made at the Constitutional Convention in 1787, by the end of the summer they’d winnowed it down to two examples: treason and bribery.

But George Mason of Virginia took issue with limiting it to the two definitions, arguing they were too narrow. At the same time the Constitution was being drafted, newspapers were covering the impeachment of a statesman named Warren Hastings for misconduct during his time the Governor General of India. Mason pointed out that under their current definition, Hasting wouldn’t be impeachable. Mason suggest they broaden the definition to include “maladministration,” meaning mismanagement or ineffective governance. James Madison argued back that the word would be too broad, and make it so the President would be serving at the “pleasure of the Senate.” He worried Senators could remove the President if they disliked a policy move.

George Mason then proposed including the phrase “high crimes and misdemeanors” instead, and that’s the term they settled on.

To understand what the framers thought “high crimes and misdemeanors” meant, Harvard Law professor Jennifer Taub points to Alexander Hamilton’s Federalist Paper No. 65, in which he explains the impeachment process. “The subjects of its jurisdiction are those offenses which proceed from the misconduct of public men, or, in other words, from the abuse or violation of some public trust,” Hamilton wrote in 1788.

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How has ‘high crimes and misdemeanors’ been used throughout American history?

The very first federal official to face impeachment was a Senator from Tennessee named William Blount. Blount had conspired to help the British conquer the Spanish-controlled territory of West Florida; the House of Representatives impeached him once he was discovered, but the Senate expelled him instead of voting on to convict him. This move by the Senate set a precedent that members of Congress aren’t impeachable under the Constitution — only federal judges and executive branch officials.

The first person who was successfully impeached and removed was federal judge named John Pickering in 1803. He was impeached because, as the University of Missouri’s Bowman says, “He was both an alcoholic and probably insane.” Bowman points out that neither was a crime, but led him to abuse his office.

Only 19 people were impeached in the U.S. from 1788 until Trump: Two Presidents, one Senator, one Secretary of War and 15 federal judges.

“These tend to be things about a violation of public trust, acting for personal gain and obstructing the process of impeachment itself,” explains Tom Ginsburg.

For an exhaustive history of impeachment, Kermit Roosevelt, a law professor at the University Pennsylvania Law School, points to the 1974 House Judiciary Committee report on the “Constitutional Grounds for Presidential Impeachment,” which was released amid the inquiry into former President Richard Nixon. The report examined the long history of impeachment — tracing it back to Britain — and concluded that “The framers understood quite clearly that the constitutional system they were creating must include some ultimate check on the conduct of the executive, particularly as they came to reject the suggested plural executive.”

One of the most important precedents the report looked at was the very first presidential impeachment.

President Andrew Johnson’s impeachment

Andrew Johnson was the first U.S. President to be impeached. Nine of the eleven articles of impeachment against him related to violating the Tenure of Office Act but, Bowman says, “The real reason was a deep disagreement between the President and Congress about reconstruction after the Civil War.”

Johnson became President after President Abraham Lincoln was assassinated. He was a unionist but also a Southern Democrat who was fine with a swift reconciliation with the South, without much social reform or protection for freed slaves. The Republican-controlled Congress deeply disagreed, and worried that Johnson was firing cabinet officials from the Lincoln Administration to replace them with officials more partial to his vision of Reconstruction. The Republicans promptly passed the Tenure of Office Act, which barred the President from firing certain executive branch officials without senatorial approval. It explicitly made the offense a “high misdemeanor.” “That was not an accident,” says Frank Bowman.

In 1868, after Johnson fired the Secretary of War, Edwin Stanton, the House of Representatives promptly impeached him. The case went to the Senate, where he came one vote away from being convicted and removed.

President Richard Nixon’s resignation

In 1974, President Richard Nixon faced impeachment charges of “high crimes and misdemeanors” after it was revealed that he used his law enforcement power to cover up a burglary of the headquarters his political opponent.

“That’s not treason and it’s not bribery, but it is a corrupt use of the powers of office in a way that undermines the constitutional system,” Richard Primus, a professor of law at the University of Michigan Law School, explains.

Three articles were approved by the Judiciary Committee but Nixon resigned before the House took the full vote. But, as Bowman explains, scholars still tend to treat the articles of impeachment brought against him as important precedent. He was charged with obstruction of justice, abuse of power and contempt of Congress. While Nixon did commit crimes, Bowman adds, none of the articles were framed in relation to the specific criminal statutes he broke. They are all framed in terms of the President’s violation of his oath of office.

A demonstration outside the Whitehouse in support of the impeachment of President Nixon (1913 - 1994) following the Watergate revelations. Getty Images

President Bill Clinton’s impeachment

President Bill Clinton was impeached in 1998 on two counts of “high crimes and misdemeanors”: lying under oath and obstruction of justice. The charges emerged after Clinton denied having had a sexual relationship with White House intern Monica Lewinsky in the course of a civil sexual harassment lawsuit against against Clinton by Paula Jones.

Stanford’s Meyler explains that the Clinton impeachment caused debate among scholars because “some people felt that, look, there’s a crime, but not every crime rises to the level of an impeachable offense. This wasn’t something that really pertained to the office, and so therefore it didn’t rise to the level of an impeachable offense.” But others argued that since a crime was clearly committed, that was enough for impeachment.

The Senate acquitted Clinton.

How has the meaning of ‘high crimes and misdemeanors’ changed over the years?

Unlike other parts of the Constitution, there’s no opportunity for the Supreme Court to interpret “high crimes and misdemeanors” and give a concrete definition. In the opinion of Erwin Chemerinsky, the Dean of the University of California Berkeley School of Law, that leaves Americans to look to how it’s been used over history. “I’d say the one thing that is shown by past practice is, it doesn’t have to be a crime,” he says. “It’s about a serious abuse of power.”

Chemerinsky argues that President Trump’s impeachment is similar to Clinton’s in that there isn’t much confusion as to what happened, but there is a great deal of disagreement about how to think about those events. The President’s own summary of his phone call shows him asking for “a favor” from the President of Ukraine, and Trump’s Chief of Staff Mick Mulvaney later said that it was a quid pro quo and told people to “get over it.” The question that remains, he believes, is whether what happens constitutes “high crimes and misdemeanors.”

Others feel differently. George Washington University Law School professor Jonathan Turley, who was called to testify about the definition of impeachable offenses by Republicans on the Judiciary Committee, testified that “the use of military aid for a quid pro quo to investigate one’s political opponent, if proven, can be an impeachable offense,” but that there is not enough evidence to prove President Trump did so.

Such disagreement is not surprising, but Chemerinsky urges observers not to panic over the discord around impeachment in Washington.

“The framers of the Constitution knew that ultimately this would be a political process,” he says. “And so none of us should be shocked or upset that it’s a political process today.”

Correction, Jan. 15

The original version of this story misstated that President Nixon ordered the burglary of the Democratic National Committee’s offices. It is still unclear if Nixon ordered or knew about the break-in before it occurred.

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Write to Madeleine Carlisle at madeleine.carlisle@time.com.