The whole body of the law is generally divided into the two classes of written law and unwritten law. Written law is the work of the legislative department of the governments; unwritten law owes its force to custom and long continued use, and its principles are to be found in the decisions rendered by the judicial department. The unwritten law in all the States, of this Country, except Louisiana, is that of the Common Law.

Law in the United States is also divided into Federal law and the law of the particular states.

The highest law of the land is the Constitution of the United States; after this comes the laws and treaties made under the authority of this Constitution. The constitutions and statutes of a state prevail over all unwritten law but yield to the Federal Constitution. In many states the written law has been codified. A code is a complete system of law, scientifically arranged and promulgated by legislative authority. A code generally includes a portion of what has formerly been the unwritten law of the state.

Municipal ordinances are the written laws passed by Municipal Corporations. Within their somewhat narrow sphere they have the force of law. Government orders and regulations, and the rules of court, while not strictly laws, have the force of laws in many respects.

The unwritten law is to be found in the reported decisions of the courts. There are complete official reports of the decisions of the Supreme Courts, both of the United States and of the States, and of many subordinate courts. It was formerly customary to give to each volume the name of its reporter. For example, the first ninety volumes of the reports of the Supreme Court of the United States were thus named. The names of these reporters with their number of volumes reported by each were as follows: Dallas, 4 (1-4); Cranch, 9 (5-13); Wheaton, 12(14-25); Peters, 16 (26-41); Howard, 24 (42-65); Black, 2 (66-67); Wallace, 23(68-90). Beginning with volume 91, the United States reports have been merely cited by the number of the volume. There has been no systematic official reports of the decisions of the inferior Federal courts. All the decisions of the Circuit Courts of Appeals and of the Circuit Courts since 1880 are contained in the Federal Reporter. Early cases are to be found in the set of books known as the Federal Cases.

Every State has its official set of reports of the decisions of the Supreme Court. In the older states the earlier volumes are named after their reporters. In some states there are official reports of the decisions of courts of a lower grade, as the Appellate Court Reports in Illinois. In addition to the official state reports there are various unofficial reports, the principal one being the different sets making up the "Reporter System" which by combining the reports of several states greatly reduces the cost to the purchaser.

In addition to American reports, the English reports may be resorted to as evidence of what the true common law principle on a certain question is. More weight is given to English reports prior to 1606 than those since that date.16