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President Obama is putting a symbolic twist on a time-honored tradition, taking the oath of office for his second term with his hand placed not on a single Bible, but two -- one owned by Martin Luther King Jr. and one by Abraham Lincoln.

The inclusion of King's Bible is particularly significant since the inauguration comes on Jan. 21, the federal holiday in honor of the civil rights leader, who delivered his "I Have a Dream" speech 50 years ago at the Lincoln Memorial. Obama will be facing the memorial as he takes the oath. King's Bible, which his children say he used early in his career as a preacher, has never been part of a presidential inauguration.

Though there is no constitutional requirement for the use of a Bible while taking the oath, George Washington began the tradition with a Bible hastily grabbed from St. John's Masonic Lodge No. 1 for his swearing-in on April 30, 1789, in New York. Since then, presidents have typically chosen Bibles with historical or personal significance, many using family heirlooms. Obama is not the first president to select two Bibles -- Harry Truman did so in 1949, Dwight Eisenhower in 1953 and Richard Nixon in 1969.

The selection of the pair of Bibles by Obama is richly symbolic of the struggle for equality in America, beginning with Lincoln's emancipation of the slaves through King's civil rights movement and ultimately to Obama becoming the nation's first black president. Inaugural planners say Obama plans to place his left hand on the stacked Bibles held by first lady Michelle Obama as he raises his right hand to repeat the oath administered by Supreme Court Chief Justice John Roberts. It hasn't been determined which will be on top with Obama's hand actually resting on it, but King's is larger so it may need to be on the bottom.

Obama used the Lincoln Bible while taking the oath four years ago -- the first time it had been used since the 16th president's inauguration in 1861. Obama's inaugural committee says that the president plans to use the first lady's family's Bible for a private swearing-in at the White House on Sunday, Jan. 20. Public presidential inaugurations traditionally aren't held on Sundays, even though the Constitution states that a president's new term begins automatically at noon on the 20th.

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King's children describe their father's King James version as his "traveling Bible" that he took as part of a collection of books he carried with him while constantly on the road and used for inspiration and preparing sermons and speeches. His daughter Bernice King says her father marked the pages with several dates from May 1954, the same month he delivered his first sermon.

"We know our father would be deeply moved to see President Obama take the oath of office using his Bible," King's children said in a statement provided by the inaugural committee. "His traveling Bible inspired him as he fought for freedom, justice and equality, and we hope it can be a source of strength for the president as he begins his second term."

Obama also plans to honor King throughout his inaugural weekend, beginning by asking Americans to volunteer in their communities on Saturday, Jan. 19, to honor the civil right leader's legacy of service. The King family plans to participate. Inaugural planners also say there will be a float honoring King in the parade to the White House after the swearing-in ceremony.

Some presidents kiss the book after completing the oath. Sometimes the Bible is open to meaningful passages, such as President George W. Bush's choice of Isaiah 40:31 -- "Those who hope in the Lord will renew their strength. They will soar on wings like eagles; they will run and not grow weary, they will walk and not be faint."

Inaugural planners say Vice President Joe Biden will use a Bible with a Celtic cross on the cover that has been in his family for 120 years. Biden has used the Bible every time he's been sworn into federal office, back to his entry in the Senate 40 years ago.