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The 35-word oath is explicitly prescribed in the Constitution, Article II, Section 1, which begins by saying the president "shall" take the oath "before he enter on the execution of his office."

The oath reads: "I do solemnly swear that I will faithfully execute the office of president of the United States and will to the best of my ability, preserve, protect and defend the Constitution of the United States."

In giving the oath, Roberts misplaced the word "faithfully," at which point Obama paused quizzically. Roberts then corrected himself, but Obama repeated the words as Roberts initially said them.