There are two basic types of legal systems used in the Western World today -- the Common Law System and the Civil Law System. The Common Law System was developed in England and used by its territories and colonies. The Civil Law System originated with the Romans and evolved with French and Spanish influences.
At present, the majority of jurisdictions in the world are civil law jurisdictions. However, the United Kingdom and former British colonies still use the common law. This includes all of the United States except Louisiana. Due to its history as a colony of both France and Spain, which have civilian systems, and its interaction with other states' common law systems, Louisiana is a mixed jurisdiction, having both common and civil law characteristics. Louisiana's legal system is essentially a civil law system that has been adapted over nearly two centuries to facilitate a harmonization with the laws of the United States.
Due to its unique legal system, Louisiana's laws are being studied by scholars from other countries that wish to joint NAFTA and the European Union. These countries seek to adapt their present civilian legal systems to fit into the common law systems of the Unites States and the United Kingdom. Louisiana is the perfect model and, some countries, such as Costa Rica, have adopted Louisiana laws verbatim.
The major difference between the common law system and the civil law system is the source of law. In common law systems, laws arise from the decisions made by judges in individual cases. Statutes in common law systems are meant to mimic and interpret judicial decisions. The civil law system is based on a "civil code," a set of laws that contain all the basic principles of the legal system. The civil code defines such things as "persons" and "property" and provides rules applicable to most situations. Judges determine the facts and interpret the provisions of the civil code as they apply to those facts.
In practice, Louisiana judges rely on interpretations of Louisiana statutes contained in published opinions of appellate courts. This body of decided cases in known as "jurisprudence" to Louisiana lawyers. The result, similar to that of common law systems, is that lawyers must research and be familiar with previously decided cases in order to give opinions and argue cases before Louisiana courts.
For a more in-depth analysis of common and civil law, please se: William Tetley: Mixed Jurisdictions: Common Law v. Civil Law (Codified and Uncodified), 600 Louisiana Law Review 677, Spring 2000.