The Pocket veto reference article from the English Wikipedia on 24-Apr-2004
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Pocket veto

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A pocket veto is a legislative maneuver in American federal lawmaking. It occurs when the President does not sign a bill into law within the amount of time allowed by the Constitution (10 days). Congress must not be in session in order for a pocket veto to take effect.

If Congress is in session and the president does not sign the bill, it becomes law without his approval. If it is not in session, the bill is killed by the pocket veto.