In order to create a new law, the Congress must go through a precise procedure specified in part by Article I, Section 7 of the Constitution. Most new legislation (called "bills") may be originally introduced in either the House of Representatives or the Senate, or identical bills may be introduced simultaneously in both chambers. (The exception is bills which include tax provisions, which must originate in the House, but can be amended as normal in the Senate.) Once a bill is introduced in a chamber, it is assigned a number (e.g., H.R. 137, S. 26) and sent to one or more committees which have jurisdiction over the subject-matter of the proposed legislation. In committee, the bill may be further distributed to sub-committees.
In the committee or subcommittee, the bill is deliberated by the committee members and may be amended by majority vote. Most bills die in committee, but some are eventually "reported out" to the full chamber—or in the case of a subcommittee, to the full committee, which can perform anouther round of edits. The committee-edited version of the proposed legislation is referred to as the "Chairman's mark." In addition, the committee will prepare a Committee Report, which analyzes the legislation and discusses why committee members feel any given provision in the marked-up bill is appropriate (or why they don't). These reports do not have the force of law, but because they show what those legislators most deeply concerned with this bill were thinking about it, the reports are frequently used by courts when attempting to interpret statutes.
Once the bill is reported reported favorably to the full chamber, it must be scheduled for deliberation by the full body. The House or Senate leadership makes scheduling decisions. Sometimes the leadership will refuse to schedule a vote on legislation which might benefit the other party, but this can be a politically risky move if it derails popular bills. Once scheduled, the bill is deliberated on the Floor of the chamber. Any member can propose amendments to be accepted by majority vote. The chamber must then pass the bill by a simple majority. Once one chamber passes a bill, it must go to and be passed by the other chamber. As noted above, bills are often introduced in both chambers simultaneously.
Whether the House and Senate consider a bill simultaneously or sequentially, because of the committee process and amendments from the Floor, the chambers usually pass significantly different versions of most bills. The two versions must be reconciled before the bill becomes a law. To do this, the bills are sent to a special ad hoc committee, called a conference committee, made up of both senators and representatives who have interest in the subject of the legislation. The conference committee harmonizes the two bills, and can add new provisions just like any other committee. If the bill survives this process, it is eventually reported back to the two chambers for final passage. The harmonized bill is then voted on by both the Senate and the House without further amendment. If the bill receives a simple majority in each chamber, it is considered to have been passed by Congress. However, it is not yet a law.
After Congress has passed a bill, it must be presented to the president before it becomes a law. Four things can happen. If the president signs the bill, it becomes law upon his signature. If he does not sign it, it will become law 10 days after it was presented to him (excluding Sundays). However, if Congress adjourns before the 10-day limit is up, the bill will not become law—this is called a "pocket veto." Finally, the president can veto the bill. If he does so, the president must return the bill to Congress within the 10-day limit and attach a statement explaining why he found the bill unacceptable.
After a bill is vetoed, Congress has three options. They can do nothing and let the bill die, they can amend the bill to address the president's concerns, or they can override his veto. If the Congress decides to amend the bill, the amendments must be passed in both chambers or the amended versions must go to conference, just like a brand-new bill. To override a veto, both chambers must repass the vetoed bill by a 2/3 majority. If they do so, the bill will become law regardless of the president's opposition.
When a bill becomes law (either with the president's signature, without it, or over his veto), it is assigned a number. For instance, the 36th public law passed by the 108th Congress (2003-2004), the Keeping Children and Families Safe Act of 2003, is denominated P.L. 108-36. Once a law is passed, it remains law until amended or repealed by Congress, which must be passed by both chambers and signed by the president just like any other legislation.