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,

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

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