Top members of the three branches of government will come together in a rare display of national unity and tradition when the presidential and vice-presidential oaths of office are delivered at Monday's inauguration. A swear-in rookie, and perhaps funny hats, will be indispensable parts of the ceremonies.

Chief Justice John Roberts and Justice Brett Kavanaugh will continue a nearly 240-year-old tradition of administering the oaths to President-elect Trump and his No. 2, JD Vance. The other seven members of the high court are expected to attend the event in the Capitol Rotunda, all in their judicial robes. 

Whatever political differences exist, they surely will not be on display at this most cordial and dignified of ceremonies. After all, the first person the president thanks will likely be the chief justice. But an undercurrent of tension remains.

During his first run for high office in 2016, candidate Trump took the unusual step of attacking a member of the federal judiciary, labeling Roberts "an absolute disaster" among other personal insults. This will be the "Chief's" fifth presidential swearing-in, his second with Trump.

HOW TO WATCH, STREAM TRUMP'S 2025 INAUGURATION ON JANUARY 20TH

The choice of Kavanaugh is no surprise: incoming second lady Usha Vance clerked for Kavanaugh when he was a judge on the U.S. Court of Appeals in Washington.

She then went on to a prestigious law clerkship at the Supreme Court with Roberts. Sources say Kavanaugh gave an especially strong job recommendation for Usha Vance to his now bench colleague.

In an August interview with "Fox and Friends," Usha Vance said Kavanaugh was "such a good boss" and "decent person" who "hired people from all over the political spectrum."

"My experience working for him was overwhelmingly positive," she added.

Sandra Day O'Connor, Ruth Bader Ginsburg and Clarence Thomas are among recent justices who have performed similar vice-presidential swear-in honors.

While chief justices have normally sworn in the president, a broader mix of officials have handled the vice-presidential duties. Then-House Speaker Dennis Hastert swore in Vice President Dick Cheney in 2005.

Thomas did the honors when Mike Pence was sworn in 2017 as vice president for Trump's first term.

TRUMP SWEARING-IN TO MOVE INDOORS DUE TO COLD WEATHER, SOURCE TELLS FOX NEWS

Article VI of the Constitution requires executive officers, including the president, as well as members of Congress and federal judges, to "be bound by oath or affirmation," but nothing mandates that a Supreme Court justice administer it. When it comes to the presidential inauguration, they just have, most of the time.

There was no Supreme Court yet formed when George Washington took the first oath of office in 1789, so New York's highest ranking judge did the honors at Federal Hall on Wall Street. Four years later, Associate Justice William Cushing swore in Washington for a second term, beginning the Supreme Court tradition.

Early swear-ins were usually conducted in the House or Senate chamber. The 1817 inaugural was held outdoors for the first time when James Monroe took the oath in front of the Old Brick Capitol, where the legislature met temporarily after the original Capitol was burned by invading British troops in the War of 1812. The Monroe swear-in site is now occupied by the Supreme Court, which opened its building in 1935.

The man who handled the duties more than 200 years ago was John Marshall, widely acknowledged as the most influential chief justice in U.S. history. He participated in a record nine swear-ins, from Thomas Jefferson to Andrew Jackson. For Roberts, this will be his fifth.

The Constitution lays out the exact language to be used in the 34-word oath of office: "I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States."

Many judges have tacked on four little words, "so help me God." It is not legally or constitutionally required, unlike other federal oaths that invoke the words as standard procedure. Historians have been at odds over whether President Washington established precedent by adding the phrase on his own during his first acceptance, but contemporary accounts mention no such ad-libbing.

Abraham Lincoln was reported to have said it spontaneously in 1861, and other presidents over the years have followed suit. A Bible is traditionally used, with the president placing one hand on it while raising the other during the oath of office.

The 16th president and Chief Justice Roger Taney shared a mutual animosity. When the oath was administered just days before the Civil War erupted, many attending the ceremony noticed the frosty demeanor both men showed each other, befitting the late winter chill. Several historians have said Lincoln later that year secretly issued an arrest warrant for Taney, who tried to block the president's suspension of habeas corpus during the conflict. The warrant was never served.

President Barack Obama used Lincoln's Bible for his two swear-ins.

Trump is expected to again use the Lincoln Bible and a family Bible.

TRUMP, VANCE OFFICIAL PORTRAITS RELEASED AHEAD OF INAUGURATION

Roberts, administering his first presidential oath in 2009, strayed slightly from the text, which prompted its re-administration for protective purposes the following day, in a private White House ceremony.

Those Jan. 20 ceremonies at the Capitol also ran long, so that the presidential oath was not completed until five minutes past noon. Nonetheless, Obama under the 20th Amendment had officially assumed the presidency at noon.

At the time, a California atheist, Michael Newdow, objected and went to federal court to prevent Roberts from prompting Obama to repeat the "so help me God" phrase. Newdow, along with several non-religious groups, argued the words violated the constitutional ban on government "endorsement" of religion.

The high court ultimately rejected the lawsuit, and no such legal challenges are expected this time.

Four years later, Justice Sonia Sotomayor swore in Biden for a second term as vice president in 2013. She was asked by Vice President Harris to do the honors again, with the first female vice president citing the fact both women once served as government prosecutors.

Pence used the family Bible of the late President Ronald Reagan, telling Fox News at the time, "It's just very humbling for me. We are approaching it with prayer, but with deep, deep gratitude to the president-elect for his confidence and deep gratitude to the American people." 

Trump also broke tradition by not attending the swear-in of his successor four years ago.

Lyndon Johnson's swear-in from 1965 marked a change from tradition. His wife Claudia – known as Lady Bird – held the Bible, a job previously managed by the high court's clerk. Spouses have since had the honor, and Melania Trump and Usha Vance are expected to continue that role.   

Hopefully, nerves won't result in a repeat of the 1941 goof, when then-clerk Elmore Cropley dropped the Bible just after Franklin Roosevelt took the oath to begin his third presidential term.

GET TO KNOW DONALD TRUMP'S CABINET: WHO HAS THE PRESIDENT-ELECT PICKED SO FAR?

It usually is not hard at the inauguration to spot the justices, who are normally shielded from broad public view in the camera-barred court. They are announced as a group, arrive wearing their black robes – usually covering bulky winter coats – and are given prominent seats on the specially built platform on the West Front of the Capitol.

Before Marshall took over the court in 1801, the justices wore red robes with fur trim and white wigs in all public settings. His practice of a simple black silk robe without wig remains the American judicial standard.

And if there is any doubt about their identities, look for some unusual-looking headgear several justices may be sporting. The large black "skullcaps" have no brims and can be made of wool, silk or even nylon. Perhaps to keep them from looking like a Jewish yarmulke, the hats are usually pleated upward, which one federal judge privately told Fox News made him look like he was wearing a dirty napkin.

Given the inauguration ceremony is indoors this year because of expected frigid weather, the skullcaps may be an afterthought.

They have been around in British courts since the 16th century, and at least a century in the United States. Only judges wear them, and only at formal ceremonies, not in court.

Official records are hazy on the hats, but Chief Justice Edward White proudly wore one in 1913 when Woodrow Wilson became president. The "age of the skullcap" peaked in 1961 when seven of the nine justices wore them at the bitterly cold inauguration of President John F. Kennedy.

The last time around, only now-retired Justice Stephen Breyer was brave enough to sport one, though Thomas, Anthony Kennedy, and the late Antonin Scalia had worn them previously. None of the six current or former women justices ever used them.

Scalia told an audience a few years ago why he favored skullcaps. "If you've ever seen an inauguration, you will see me wearing the old hats judges used to wear. It's a ridiculous-looking hat, but it's a tradition. Yes, it's silly looking."

Scalia's headgear was a replica of one worn by St. Thomas More, a gift from the St. Thomas More Society of Richmond, Virginia

The late Chief Justice William Rehnquist also sported them, not surprising, given his role as an unofficial historian of court procedure and tradition.

He made one of the most dramatic appearances in inaugural history while suffering from thyroid cancer in 2005. There was speculation he would be too ill to attend, but he assured officials he would be there, and he kept his word.

After three months away from the public eye while he received chemotherapy, the ailing 81-year-old chief was introduced to the audience just before President George W. Bush was to take the oath. Using a cane, Rehnquist walked slowly to the podium without assistance – wearing a dark baseball cap – and did the honors. His voice was clear but raspy, because of a trachea tube in his throat, which was hidden by a scarf.

Afterward, Rehnquist wished Bush good luck, then was quickly escorted out of the cold.

Rehnquist also swore in President Bill Clinton eight years earlier. Unbeknownst to Clinton or the public, the justices days earlier had taken a private vote in Clinton v. Jones. Their ruling said the president could not refuse to testify in an ongoing civil lawsuit against him by Paula Jones, who alleged sexual harassment. That triggered a series of events leading to Clinton's 1999 impeachment trial in the U.S. Senate, presided over by Rehnquist himself, without the skullcap.