At his request, President Barack Obama is ending his inaugural oath with: “So help me God.”
Those four words are not legally or constitutionally required, unlike other federal oaths that invoke them as standard procedure.
Historians have wrangled over whether George Washington established precedent by adding the phrase on his own during his first Inaugural acceptance, but the Library of Congress website states he did.
Abraham Lincoln was reported to have said it spontaneously in 1861 and other presidents over the years have followed suit.
A Bible is traditionally used in administering the oath.
Obama took the official oath on Sunday at the White House with his left hand on the family Bible of his wife, Michelle.
At Monday’s ceremonial swearing-in at the Capitol, he will use Bibles from Lincoln and Martin Luther King Jr.