Onlookers were surprised at President Trump’s inaugural oath when he did not put his hand on a Bible. Melania was holding a stack of two Bibles during the oath-taking ceremony, and while the president raised his right hand, his left hand remained at his side as he repeated the oath recited by Chief Justice John Roberts.

The oath was legally (and morally) binding as per Article VI of the U.S. Constitution, which states that “all executive and judicial Officers, both of the United States and of the several States, shall be bound by Oath or Affirmation, to support this Constitution; but no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States.”

Article Two of the Constitution says an incoming president “shall take the following Oath or Affirmation:- 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.”

As the oath was binding, the president did not have to redo the swearing-in.  In 2009,  Supreme Court Chief Justice John Roberts and President Barack Obama stumbled through the words of the oath, prompting a non-public redo.

The two Bibles Trump had chosen beforehand were his personal Bible gifted to him by his mother and the Lincoln Bible that President Abraham Lincoln used to take the oath of office in 1861.

During his first inauguration on Jan. 20, 2017, Trump placed his right hand atop two Bibles held by the first lady as Roberts swore him in as the 45th president.

Trump is not the first president to take the oath of office without placing his hand on a Bible. At least four presidents have taken the oath without touching a Bible.

According to the Joint Congressional Committee on Inaugural Ceremonies, the nation’s sixth president, John Quincy Adams, was sworn into office in 1825 while reading a law book. President Theodore Roosevelt also did not use a Bible when he took the oath of office in 1901 in the aftermath of the assassination of President William McKinley, according to the committee.

In the wake of President John F. Kennedy’s assassination in 1963, President Lyndon B. Johnson took the oath of office aboard Air Force One by placing his left hand on a Catholic missal, or prayer book, according to the LBJ Presidential Library.

In his autobiography, President Calvin Coolidge, the nation’s 30th president, claimed he did not use a Bible for the oath. “The oath was taken in what we always called the sitting room, by the light of the kerosene lamp, which was the most modern form of lighting that had then reached the neighborhood,” Coolidge wrote. “The Bible which had belonged to my mother lay on the table at my hand. It was not officially used, as it is not the practice in Vermont or Massachusetts to use a Bible in connection with the administration of an oath.”