President Donald Trump swore his oath of office as president for the second time in the Capitol rotunda, one hand raised in the air, but declined to place his other hand on the two Bibles held before him by First Lady Melania Trump.

While there is no constitutional requirement to swear an oath by religious or secular text, to do so has become customary for nearly all U.S. officeholders.

According to Article VI, Clause 3 of the U.S. Constitution, members of Congress, state legislatures, and executive and judicial officers are bound "by oath or affirmation" to support the Constitution.

It also states that "no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States."

As such, some non-religious officials have sworn the oath empty-handed or with some other document, such as a copy of the Constitution itself. However, it not known why Trump, who has proclaimed his Christian faith and even supported the rollout of his own edition of the Bible, elected not to place his hand on the family Bible and so-called Lincoln Bible, the latter used by the 16th president in 1861 and former President Barack Obama in 2009 and 2013.

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