On Feb. 8, the Senate will begin the second impeachment trial of former President Donald J. Trump. This trial is unprecedented for two reasons: it’s the only time a president has been impeached twice, and it’s the first impeachment trial to occur after a president has left office. But that doesn’t mean unusual or unprecedented impeachments have never happened before. Their history offers a helpful guide for understanding the coming weeks and the constitutional basis for the looming trial.

So many of the legal limitations on the presidency are theoretical and unwritten because they have never been tested. The country is the longest-surviving democracy in the world, but only 45 men have held the highest office (there have been 46 administrations, as Grover Cleveland served two, non-concurrent terms). Forty-five individuals is a relatively small sample size and presidential behavior generally conforms to certain norms and standards, so Americans don’t have to consider new or unprecedented legal questions all that often. We saw this principle tested in the final weeks of President Trump’s administration as he considered a self-pardon. The Constitution doesn’t explicitly address a self-pardon, so legal experts debated its legality, but there will be no definitive answer until an individual attempts to assert this power and it’s challenged in court.

The Constitution also doesn’t explicitly explain the process for impeaching a former president, and it has never been addressed until now. Presidential behavior has prompted the House of Representatives to vote on articles of impeachment only four times in 230 years. Out of those four instances, only one — Trump’s second impeachment — occurred late enough in the administration to raise the question of impeachment after the end of a president’s term in office.

When posed with a constitutional question like this one, both historians and legal experts look to parallels in history that might provide some guidance. Here are a few helpful examples that shed light on this tricky issue.

First, the impeachment of Judge West Humphreys might be the most accurate comparison. By all accounts, Humphreys was a perfectly competent lawyer, attorney general of Tennessee, and U.S. district judge for the Eastern, Western, and Central Districts of Tennessee. President Franklin Pierce had nominated Humphreys to the judgeship on March 24, 1853, and he might have served peacefully until he retired or died, if not for the Civil War.

In June 1861, Tennessee seceded from the Union and joined the Confederacy. Accordingly, the Confederate government created its own District Court for the District of Tennessee and Jefferson Davis (president of the Confederacy), nominated Humphreys as judge. Humphreys accepted this position, but also refused to relinquish his role as the Union district judge for the districts of Tennessee. While Humphreys had no desire to participate in the Union, he also refused to resign in order to prevent a pro-Union judge from taking the seat.

Needless to say, Congress didn’t take too kindly to that decision. On May 19, 1862, the House of Representatives voted to impeach Humphreys on six charges, including that he “endeavor[ed] by public speech to incite revolt and rebellion within said State against the Constitution and Government of the United States.” On June 26, the Senate began a quick trial and unanimously convicted Humphreys on five of the six charges that very afternoon. He was removed from office and barred from holding future office.

Next, in March 1876, the House of Representatives impeached former Secretary of War William Belknap. Belknap is the first and only cabinet secretary to be impeached, but more importantly for our case, the Senate trial commenced after Belknap left office. The saga actually began in 1870, not long after Belknap took office. He gave Caleb Marsh and Marsh’s associates the rights to operate a lucrative military trading post at Fort Sill in Oklahoma. For the next five years, Marsh and his associates made a fortune and sent quarterly payments to Belknap, totaling over $20,000 (almost $400,000 in 2021 dollars).

Belknap brought about his own downfall through his obviously ostentatious lifestyle. He did not inherit family wealth and his $8,000 government salary didn’t go far, yet he hosted extravagant parties in Washington, D.C., and his wife was always decked out in the newest fashions. In early 1876, a House committee opened an investigation and uncovered evidence of the War Department corruption.

On March 2, 1876, the House was scheduled to vote on articles of impeachment. Minutes before the vote began, Belknap rode to the White House and delivered his immediate resignation to President Ulysses S. Grant. The resignation didn’t stop the House and five days later, the representatives unanimously voted to send the articles to the Senate. The House accused Belknap of “criminally disregarding his duty as Secretary of War and basely prostituting his high office to his lust for private gain.”

In April, the Senate gathered and agreed that it retained jurisdiction over the impeachment process, even though Belknap had left office. Over the next month, it called more than 40 witnesses. On Aug. 1, 1876, a majority of senators voted in favor of conviction for all five articles of impeachment. However, the Senate vote didn’t reach the two-thirds majority and Belknap was acquitted.

There are more recent examples as well. On Aug. 3, 1990, President George H.W. Bush nominated Samuel B. Kent as a judge on the U.S. District Court for the Southern District of Texas. In August 2007, the chief judge of the Southern District announced that Kent would not be hearing cases between September 2007 and January 2008. In August 2008, the reason for Kent’s absence came to light when he was indicted on three counts of abusive sexual contact and aggravated sexual abuse. In February 2009, Kent pled guilty to obstruction of justice, agreed to retire, and was sentenced to 33 months in prison on May 11, 2009.

Kent’s retirement, salary, and pension prompted impeachment charges. When pleading guilty, Kent intended to retire, but he was only 59 years old. Federal statute provides pensions for judges that serve to 65 years of age. Judges with permanent disabilities can receive early retirement, but in May 2009 the U.S. Court of Appeals denied Kent’s disability status and recommended impeachment. In order to try and avoid impeachment, Kent submitted his resignation to President Obama, effective June 1, 2010. In the meantime, Kent continued to draw his salary and would have continued to do so until the following year.

The House would not tolerate that compromise. On June 19, representatives approved the articles of impeachment and sent them to the Senate. A few days later, a few officials visited Kent in prison to serve him with impeachment trial summons. Kent offered a new resignation letter that would be effective on June 30. With Kent’s immediate removal in sight and criminal consequences already in force, Congress agreed to end the impeachment proceedings.

Each of these cases offers valuable insight for the upcoming impeachment proceedings and demonstrates that there is no such thing as a typical impeachment. Indeed, history — especially presidential history — is still being written every day.