<P> All legislation passed by both houses of Congress must be presented to the President . This presentation is in the President's capacity as Head of Government . </P> <P> If the President approves of the legislation, then the President signs it into law . According to Article 1 . Section 7 of the Constitution, when the President chooses not to sign or does not approve of the bill, the President must return the bill, unsigned, within ten days, excluding Sundays, to the house of the United States Congress in which it originated, while the Congress is in session . The President is constitutionally required to state any objections to the bill in writing, and the Congress is constitutionally required to consider them, and to reconsider the legislation . This action, in effect, is a veto . </P> <P> If the Congress overrides the veto by a two - thirds vote in each house, it becomes law without the President's signature . Otherwise, the bill fails to become law unless it is presented to the President again and the President chooses to sign it . Historically, the Congress overrides the Presidential veto 7% of the time . </P> <P> A bill can also become law without the President's signature if, after its presentment, the President simply fails to sign it within the ten days noted . If there are fewer than ten days left in the session before Congress adjourns, and if Congress does so adjourn before the ten days have expired in which the President might sign the bill, then the bill fails to become law . This procedure, when used as a formal device, is called a pocket veto . </P>

What principle of the constitution allows congress to override a presidential veto
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