The law of the United States is derived primarily from four sources. These sources are constitutional law, statutory law, administrative regulations, and the common law (which includes case law).
The United States Constitution, which determines the boundaries of federal law, is the most important source of law in US. The Constitution and federal law are called “the supreme law of the land,” and as “the supreme law” they preempt conflicting state and territorial laws. However, the scope of federal preemption is limited, because the scope of federal power is limited to authority specifically provided in the Constitution. As such, many states may grant their citizens broader rights than those granted through federal Constitution, as long as they do not infringe upon any federal constitutional rights. Thus, most U.S. law consists primarily of state law, which can vary greatly in each state.
States also have delegated lawmaking powers to multitude of agencies, municipalities, counties, and special districts. All the state constitutions, statutes and regulations (as well as all the ordinances and regulations promulgated by local entities) are subject to judicial interpretation.
The fifty American states are separate sovereigns with their own state constitutions, state governments, and state courts (including state supreme courts). They retain plenary power to make laws covering anything not preempted by the federal Constitution, federal statutes, or international treaties ratified by the Senate. Normally, state supreme courts are the final interpreters of state constitutions and state law, unless their interpretation itself presents a federal issue, in which case a decision may be appealed to the U.S. Supreme Court which is the highest judicial body in the United States.
The law of most of the states is based on the common law of England. However, the passage of time has led to state courts and legislatures expanding, overruling, or modifying the common law, eventually resulting in our modern American law. As a consequence, the laws of any given state usually differ from the laws of its sister states. That is why when you are looking for an attorney you have to talk to someone licensed to practice in your state. For example, it may not be wise for you to get advice from a New York attorney if you are getting divorced in New Jersey. Despite the land proximity matrimonial laws are quite different in those two states.
Interestingly, it should be noted that some states in the southwest that were originally Mexican territories have inherited several unique features from the civil law that governed when they were part of Mexico. These states include Arizona, California, Nevada, New Mexico, and Texas.
Much of Louisiana law is derived from French and Spanish civil law, as that state used to be a colony of both France and Spain. Puerto Rico, also a former Spanish colony, is also a civil law jurisdiction. However, the criminal law of both these jurisdictions has been modified by common law influences and the supremacy of the federal Constitution. Furthermore, Puerto Rico is also unique in that it is the only U.S. jurisdiction in which the everyday working language of court proceedings, statutes, regulations, and case law is Spanish. All states, the federal government, and most territories use American English as their working language.
Traditionally, lawyers also distinguish between procedural law and substantive law. Procedural law is divided into criminal procedure and civil procedure (which governs process in all judicial proceedings involving lawsuits between private parties). Substantive law is what most people think of actual law. It comprises the actual “substance” of the law, that is, the law that defines legally enforceable rights and duties, and what wrongful acts amount to violations of those rights and duties. While substantive law by definition is vast, the five major components of American substantive law are: constitutional law, criminal law, contract law, tort law (civil wrongdoing), and property law. Simply put, other areas of law are a combination of those listed above. For example in family law, spousal abuse can be a tort or criminal matter, property settlements refer to property law, and prenuptial agreements are simply contracts.