Background
At both the federal and state levels, the law of the United States was mainly derived from the common law system of English law , which was in force at the time of the Revolutionary War. However, U.S. law has diverged greatly from its English ancestor both in terms of substance and procedure. It has incorporated a number of civil law innovations.
Royal Courts of Justice
The neo-medieval pile of the Royal Courts of Justice on G.E. Street, The Strand, London.
American Common Law
The United States and most Commonwealth countries are heirs to the common law legal tradition of English law. Certain practices traditionally allowed under English common law were specifically outlawed by the Constitution, such as bills of attainder and general search warrants.
As common law courts, U.S. courts have inherited the principle of stare decisis. American judges, like common law judges elsewhere, not only apply the law, they also make the law. Their decisions in the cases before them became the precedent for decisions in future cases.
The actual substance of English law was formally received into the United States in several ways. First, all U.S. states except Louisiana have enacted "reception statutes" which generally state that the common law of England (particularly judge-made law) is the law of the state to the extent that it is not repugnant to domestic law or indigenous conditions. Some reception statutes impose a specific cutoff date for reception, such as the date of a colony's founding, while others are deliberately vague. Therefore, contemporary U.S. courts often cite pre-Revolution cases when discussing the evolution of an ancient judge-made common law principle into its modern form. An example is the heightened duty of care that was traditionally imposed upon common carriers.
Federal courts lack the plenary power possessed by state courts to simply make up law. State courts are able to do this in the absence of constitutional or statutory provisions replacing the common law. Only in a few limited areas, like maritime law, has the Constitution expressly authorized the continuation of English common law at the federal level (meaning that in those areas federal courts can continue to make law as they see fit, subject to the limitations of stare decisis).
Federal Precedent
Unlike the states, there is no plenary reception statute at the federal level that continued the common law and thereby granted federal courts the power to formulate legal precedent like their English predecessors. Federal courts are solely creatures of the federal Constitution and the federal Judiciary Acts. However, it is universally accepted that the Founding Fathers of the United States, by vesting judicial power into the Supreme Court and the inferior federal courts in Article Three of the United States Constitution, vested in them the implied judicial power of common law courts to formulate persuasive precedent. This power was widely accepted, understood, and recognized by the Founding Fathers at the time the Constitution was ratified. Several legal scholars have argued that the federal judicial power to decide "cases or controversies" necessarily includes the power to decide the precedential effect of those cases and controversies.
State Law
The passage of time has led to state courts and legislatures expanding, overruling, or modifying the common law. As a result, the laws of any given state invariably differ from the laws of its sister states. Therefore, with regard to the vast majority of areas of the law that are traditionally managed by the states, the United States cannot be classified as having one legal system. Instead, it must be regarded as 50 separate systems of tort law, family law, property law, contract law, criminal law, and so on. Naturally, the laws of different states frequently come into conflict with each other. In response, a very large body of law was developed to regulate the conflict of laws in the United States.
All states have a legislative branch which enacts state statutes, an executive branch that promulgates state regulations pursuant to statutory authorization, and a judicial branch that applies, interprets, and occasionally overturns state statutes, regulations, and local ordinances.
In some states, codification is often treated as a mere restatement of the common law. This occurs to the extent that the subject matter of the particular statute at issue was covered by some judge-made principle at 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.