A bill must be agreed to by both chambers in the same form before it can be presented to the President. (Notably, the U.S. Constitution requires that any bill with revenue provisions must be a House bill. With this exception, it does not matter if a bill is passed first by the Senate or if it is passed first by the House.) Once one chamber passes a bill, it isengrossed – that is, prepared in official form – and then sent (or messaged) to the other chamber. In a majority of cases, the second chamber simply agrees to the exact text passed by the first chamber, in which case Congress has then completed its action on the bill.
Once both chambers of Congress have each agreed to the bill, it is enrolled – that is, prepared in its final official form and then presented to the President. Beginning at midnight on the closing of the day of presentment, the President has ten days, excluding Sundays, to sign or veto the bill. If the bill is signed in that ten-day period, it becomes law. If the president declines to either sign or veto it – that is, he does not act on it in any way – then it becomes law without his signature (except when Congress has adjourned under certain circumstances). If the President vetoes the bill, it is returned to the congressional chamber in which it originated; that chamber may attempt to override the president’s veto, though a successful override vote requires the support of two-thirds of those voting. If the vote is successful, the other chamber then decides whether or not to attempt its own override vote; here, as well, a successful override vote requires two-thirds of voting members to agree. Only if both chambers vote to override does the bill becomes law notwithstanding the President’s veto. A successful override of a presidential veto is rare.