As millions watched President Donald Trump’s inauguration at the White House on Monday, Jan. 20, many noticed that he did not place his left hand on a Bible while being sworn in. Now people are questioning that gesture, and wondering if the president can be sworn in without using a Bible.

President Trump did not place his hands on a stack of Bibles while taking the oath of office Monday, but he did not violate the U.S. Constitution either.

The tradition of swearing the oath of office on a Bible stretches back to George Washington, but not all presidents have observed it.