Biden AdministrationUkraine InvasionInflationEnergyCybersecurity

The Chief Justice and impeachment: John Roberts’ role in the Trump trial

January 21, 2020, 6:05 PM UTC

For only the third time in history, the Senate will hold a trial of a sitting President Tuesday afternoon. The process will give rise to the unusual spectacle of Chief Justice John Roberts leaving the Supreme Court to preside over the chamber of another branch of government.

How will Roberts, a conservative appointed by President George W. Bush, influence the proceedings? Here is the Chief Justice’s role in the trial explained in plain English.

Why is the Chief Justice there in the first place?

It’s in the Constitution. Article I, Section 3 states “When the President of the United States is tried, the Chief Justice Shall Preside.”

Does this mean Roberts will suspend his duties at the Supreme Court?

Nope. Roberts will instead perform double-duty, attending to Supreme Court business in the morning, which will include hearing cases, and then heading to the Senate, where the impeachment trial will start at 1 p.m. every day.

In the unexpected event that Roberts must miss any Supreme Court proceedings, Justice Clarence Thomas, who is next in seniority, will act in his role, according to the court’s Public Information Officer.

Will Roberts’ conservative views shape the outcome of the trial?

Probably not. As law professor Michael Gerhardt explains, the Chief Justice’s role is to “preside”—a largely symbolic duty. Unlike at the Supreme Court, where his power is immense, Roberts will mostly be an onlooker over what is a political process more than a legal one. Even when it comes to the rules of the trial, the senators will be able to overrule him.

Although he will be wearing the same robes he does at the Supreme Court, his main role is to act as a figurehead and bring gravitas to the proceedings. While Roberts could try to flex his legal authority, Gerhardt says this is unlikely because the Chief Justice, who has described his job as that of an impartial umpire, does not to do anything that could lead the court to be perceived as political. Also, unlike hearings at the Supreme Court, this trial will be on TV.

How did the Chief Justices perform in previous impeachment trials?

In 1868, during the Senate trial of President Andrew Johnson, Supreme Court Justice took an active role in shaping the proceedings, reportedly ruling on questions of evidence and witness eligibility (issues that are hot button topics in Trump’s trial but which Roberts is unlikely to touch).

Roberts is more likely to follow the example of Chief Justice William Rehnquist, who presided over President Clinton’s trial in 1996, and under whom Roberts served as a law clerk. Rehnquist played a minimal role in the Clinton trial, relying on Senate Parliamentarians for guidance when questions arose.

According to veteran Supreme Court watcher Nina Totenberg, Rehnquist became grumpy about the many unplanned interruptions and delays during the trial—events that did not happen at the Supreme Court—until the Senate’s sergeant-at-arms placated him with cookies and ice cream. The bored Rehnquist also resorted to playing poker with his law clerks until he was informed that was illegal. It remains to be seen what Roberts will do to stifle his frustration if the Senate hearings became tedious.

Correction, January 28, 2020: An earlier version of this story misstated President Andrew Johnson’s name.

More must-read stories from Fortune:

—Donald Trump and the power of incumbency
—Why the U.K. is bailing out regional airline Flybe when it’s let others fail
—The wealthiest members of Congress—and how they made their millions
Millions have been purged from voter rolls—and may not even realize it
2020 Crystal Ball: Predictions for the economy, politics, technology, and more
Get up to speed on your morning commute with Fortune’s CEO Daily newsletter.