The tradition of using a Bible during the swearing-in goes back to the very first inauguration, but not all Presidents have used one.

The president-elect eschewed the star-spangled “Trump Bible” in favor of volumes with personal and national history.

There are so many traditions on Inauguration Day, but only one is required by law, and that is saying the Oath of Office. Taking the Oath with your hand on a book According to the U.S. Department of State,

There’s a unique family backstory behind the holy book that the President-elect will use for his second inauguration on Monday.

Images from the inauguration show President Trump did not put his hand on a Bible as he was sworn in by Supreme Court Chief Justice John Roberts. It is not required in the Constitution.

It’s traditional to use a Bible during the presidential oath of office, but it is not required. Only the oath is mandated by the Constitution.

The placement of Trump’s left hand, however, has no bearing on the legitimacy of his first day back at the job. The U.S. Constitution does not stipulate that a president-to-be place his or her hand on a Bible when taking the Oath,

President Donald Trump did not place his hand on the Bible as he took the oath of office during his inauguration on Monday. First lady Melania Trump stood next to the president holding two Bibles, but the 47th president of the United States didn't place his hand on either as he raised his right hand to take the presidential oath,