Law of the United States
The law of the United States comprises many levels of codified and uncodified forms of law, of which the most important is the United States Constitution, which prescribes the foundation of the federal government of the United States, as well as various civil liberties. The Constitution sets out the boundaries of federal law, which consists of Acts of Congress, treaties ratified by the Senate, regulations promulgated by the executive branch, and case law originating from the federal judiciary. The United States Code is the official compilation and codification of general and permanent federal statutory law.
Federal law and treaties, so long as they are in accordance with the Constitution, preempt conflicting state and territorial laws in the 50 U.S. states and in the territories. However, the scope of federal preemption is limited because the scope of federal power is not universal. In the dual-sovereign system of American federalism (actually tripartite because of the presence of Indian reservations), states are the plenary sovereigns, each with their own constitution, while the federal sovereign possesses only the limited supreme authority enumerated in the Constitution. Indeed, states may grant their citizens broader rights than the federal Constitution as long as they do not infringe on any federal constitutional rights. Thus, most U.S. law (especially the actual "living law" of contract, tort, property, criminal, and family law experienced by the majority of citizens on a day-to-day basis) consists primarily of state law, which can and does vary greatly from one state to the next.
At both the federal and state levels, with the exception of the state of Louisiana, the law of the United States is largely derived from the common law system of English law, which was in force at the time of the American Revolutionary War. However, American law has diverged greatly from its English ancestor both in terms of substance and procedure and has incorporated a number of civil law innovations.