President Obama’s inaugural planning committee will announce Friday morning that — surprise! — Chief Justice John G. Roberts Jr. will deliver the oath of office to Mr. Obama when he is sworn in for a second term later this month.

Most Americans simply assume the chief justice always delivers the oath. In fact, the president gets to choose who will swear him in, and Mr. Obama has selected the chief justice to deliver not one, but two, oaths to him: first in a small official ceremony at the White House at noon on Sunday, Jan. 20, the constitutionally mandated date and hour for the swearing-in, and again as part of the public inaugural festivities scheduled for Monday, Jan. 21.

“I will be honored to again stand on the inaugural platform and take part in this important American tradition,’’ Mr. Obama said in an announcement the planners will release later Friday morning. “I look forward to having Chief Justice John Roberts administer my oath of office as we gather to celebrate not just a president or a vice president, but the strength and determination of the American people.”

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