Melissa Haynes, age 14, of Monroe, La., for her question:

WHAT IS COMMON LAW?

Common law is a term used to refer to the main body of English unwritten law that evolved from the 12th Century onward. The name comes from the idea that English medieval law, as administered by the courts of the realm, reflected the "common" customs of the kingdom. This system of law prevails also in the United States and Canada today.

The common law is based on the principle of deciding cases by reference to previous judicial decisions, rather than to written statutes drafted by legislative bodies. Common law can be contrasted with the civil law system, based on ancient Roman law, found in continental Europe and elsewhere.

Whereas civil law judges resolve disputes by referring to statutory principles arrived at in advance, common law judges focus more intently on the facts of the particular case to arrive at a fair and equitable result for the litigants.

As the number of judicial decisions accumulate on a particular kind of dispute, general rules or precedents emerge and become guidelines for judges deciding similar cases in the future.

Subsequent cases, however, may reveal new and different facts and considerations, such as changing social or technological conditions. A common law judge is then free to depart from precedent and establish a new rule or decision, which sets a new precedent as it is accepted and used by different judges in other cases. In this manner, common law retains a dynamic for change.

In all common law systems, a pyramidal structure of courts exists to define and refine the law. At the base of the pyramid are trial courts, composed of a single judge and a jury selected from among the local citizens.

Above the trial courts, layers of appellate courts, composed entirely of judges, exist to adjudicate disputes. These decide if the trial judge applied the correct principles of law. Common law has been known as unwritten law, because it is not collected in a single source. Reports of the judicial decisions from which the common law was derived were only occasionally circulated from the 12th to the 16th century.

During the 17th century, more formal reports of decisions were published by private parties. These reports became increasingly accurate and complete until the 19th century. At that time, the courts themselves took over the responsibility for publishing judicial decisions. It is primarily the decisions of appellate rather than trial courts that are published.

Most of the British common law as it existed at the time of the American Revolution was received into the courts of the original states, becoming the foundation of a distinctly American system of law. Only one state, Louisiana, differs significantly from the rest, basing its state law system on the French civil law model.

Common law has varied from state to state as conditions and local customs have varied.

This is the order of authority of law in the U.S.: the Constitution, treaties with foreign powers and acts of Congress, state constitutions, state statutory law and, finally, the common law.