Law of the United States
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The law of the United States was originally largely derived from the common law of the system of English law, which was in force at the time of the Revolutionary War. However, the supreme law of the land is the United States Constitution and, under the Constitution's Supremacy Clause, laws enacted by Congress and treaties to which the U.S. is a party. These form the basis for federal laws under the federal constitution in the United States, circumscribing the boundaries of the jurisdiction of federal law and the laws in the fifty U.S. states and territories.
Sources of law
In the United States, the law is derived from four sources. These four sources are constitutional law, administrative law, statutes, and the common law (which includes case law). The most important source of law is the United States Constitution, and everything falls under, and is subordinate to, it. No law may contradict the United States Constitution. For example, if Congress passes a statute that conflicts with the constitution, the Supreme Court may find that law unconstitutional, and strike it down.
American common law
Although the United States and most Commonwealth countries are heirs to the common law legal tradition of English law, American law tends to be unique in many ways. This is because the American legal system was severed from the British system by the Revolution, and afterwards, it evolved independently from the British Commonwealth legal systems. Therefore, when attempting to trace the development of traditional judge-made common law principles, that is, the few that have not already been overridden by newer laws, American courts will look at British cases only up to the early 19th century.
Although the courts of the various Commonwealth nations are often influenced by each other's rulings, American courts rarely follow post-Revolution Commonwealth rulings unless there is no American ruling on point, the facts and law at issue are nearly identical, and the reasoning is strongly persuasive. The earliest American cases, even after the Revolution, often did cite contemporary British cases, but such citations gradually disappeared during the 19th century as American courts developed their own principles to resolve the legal problems of the American people. Today, the vast majority of American legal citations are to domestic cases. Sometimes, courts, and casebook editors, do make exceptions for opinions on issues of first impression by brilliant British jurists, like William Blackstone or Lord Denning.
Many of these early decisions and common law principles were made irrelevant or invalidated by the passage of codes and revised statutes in the 20th century.
Federal law in the United States originates with the Constitution, which gives Congress the power to enact statutes for certain limited purposes like regulating commerce. Nearly all statutes have been codified in the United States Code. Many statutes give executive branch agencies the power to create regulations, which are published in the Code of Federal Regulations and also carry the force of law. Many lawsuits turn on the meaning of a federal statute or regulation, and judicial interpretations of such meaning carry legal force under the principle of stare decisis.
The fifty American states are separate sovereigns with their own constitutions and retain plenary power to make laws covering anything not preempted by the federal Constitution or federal statutes. Nearly all the original states and those created by the Louisiana Purchase had the same British common law base. (Louisiana law itself was strongly influenced by the French Napoleonic Code.) However, the passage of time has resulted in enormous diversity in the laws of the states. Over time, state courts expanded the old common law rules in different directions (through their traditional power to make law under stare decisis), and state legislatures passed various statutes expanding or overriding such judge-made rules.
Unlike other common law jurisdictions, all American states have codified some or all of their statutory law into legal codes, which was an idea borrowed from the civil law through the efforts of American lawyer David Dudley Field. New York's codes are known as "Laws." California and Texas simply call them "Codes." Most other states use "Revised Statutes," "Compiled Statutes," or some other name for their codes. California, New York, and Texas have separate subject-specific codes, while all other states and the federal government use a single code divided into numbered titles.
In some states, codification is often treated as a mere restatement of the common law. Judges are free to liberally interpret the codes unless and until their interpretations are specifically overridden by the legislature. In other states, there is a tradition of strict adherence to the plain text of the codes.
The advantage of codification is that once the state legislature becomes accustomed to writing new laws as amendments to an existing code, then the code will usually always reflect democratic sentiment as to what the current law is.
In contrast, in jurisdictions with uncodified statutes, like the United Kingdom, it is much harder to determine what the current law is. One has to trace back to the earliest relevant Act of Parliament, and then identify all later Acts which purported to amend the earlier Act or which directly overrode it. For example, when the UK decided to create a Supreme Court of the United Kingdom, it had to identify every single Act referring to the House of Lords that was still good law, and then amend all of them to refer to the Supreme Court.
However, American codes are not the direct equivalents of their civil law counterparts (see civil code), and should not be confused with them. American codes do not have complete internal logical coherence nor do they currently aspire to such a status.  Some states attempted to reconcile the old common law lawmaking model with the concept of codification via compromise. For example, California codes are to be liberally construed as a continuation of the common law to the extent that they harmonize with the common law or pre-code statutes. 
In the arena of criminal law, all states have somewhat similar laws in regard to "higher crimes," such as murder and rape, although penalties for these crimes may vary from state to state. Additionally, state laws dealing with drug crimes vary widely, with some states treating possession of small amounts of drugs as a misdemeanor offense or as a medical issue and others categorizing the same offense as a serious felony.
However, for public-welfare offenses where the state is punishing merely risky (as opposed to injurious) behavior, there is significant diversity across the various states. For example, the laws controlling drunk driving were rather unstandardized prior to the 1990s.
United States tort law for personal injury tends to vary widely across the states. For example, a few jurisdictions allow actions for negligent infliction of emotional distress even in the absence of physical injury, but most do not.
Attempts at "uniform" laws
Efforts by various organizations to create "uniform" state laws have been only partially successful. The two leading organizations are the American Law Institute (ALI) and the National Conference of Commissioners on Uniform State Laws (NCCUSL). The most successful and influential uniform laws are the Uniform Commercial Code (a joint ALI-NCCUSL project) and the Model Penal Code (from ALI).
Apart from model codes, the American Law Institute has also created Restatements of the Law which are widely used by lawyers and judges as substitutes for long, tedious citations of old cases (in order to invoke the long-established principles contained in those cases).
In areas where state laws conflict, there are often "majority" and "minority" rules. A majority rule is a principle followed by a majority of the states. For example, in medical malpractice cases involving minor patients, most states have decided that plaintiffs, in order to show a lack of informed consent, must show there is information that should have been provided and was not, and that if it had been provided, a reasonable person in the parents' position would not have consented. The miniority rule, by contrast, requires plaintiffs only to show that the actual parents would not have consented.
States have delegated lawmaking powers to a staggering number of agencies, counties, cities, and special districts. And all the state constitutions, statutes and regulations are subject to judicial interpretation like their federal counterparts.
Thus, at any given time, the average American citizen is subject to the rules and regulations of several dozen different agencies at the federal, state, and local levels, depending upon one's current location and behavior.
Unlike the rest of the country, as noted above, state law in Louisiana is based on the Napoleonic Code, inherited from its time as a French colony. Puerto Rico is also a civil law jurisdiction. However, the criminal law of both jurisdictions has been necessarily modified by common law influences and the supremacy of the federal Constitution.
California is a common law jurisdiction with a few features borrowed from the civil law. Besides the codification noted above, it has a community property system for the property of married persons. Also, the California Civil Code shows civil law influences in that the law of contracts is treated as part of the law of obligations (though the rules actually codified are clearly derived from the common law).
Acts and Codes
- United States Code
- Controlled Substances Act
- Digital Millennium Copyright Act
- Sonny Bono Copyright Term Extension Act
- Sudan Peace Act
- False Claims Act
- List of sources of law in the United States
- List of United States Supreme Court cases
- List of Uniform Acts (United States)
- List of United States federal legislation
- Texts of US federal laws and US state laws
- U.S. Code collection at Cornell University's Legal Information Institute
- Elizabeth Gaspar Brown, "Frontier Justice: Wayne County 1796-1836," in Essays in Nineteenth-Century American Legal History, ed. Wythe Holt, 676-703 (Westport, CT: Greenwood Press, 1976): 686. Between 1808 and 1828, the briefs filed in court cases in the Territory of Michigan changed from a complete reliance on English sources of law to an increasing reliance on citations to American sources.
- Constitutional Reform Act 2005, via Office of Public Sector Information (OPSI.Gov.uk)
- Andrew P. Morriss, "Codification of the Law in the West," in Law of the Western United States, ed. Gordon Morris Bakken, 45-54 (Norman: University of Oklahoma Press, 2000), 53.
- Morriss, 48.