There are generally considered to be five legal systems in the world today: civil law, common law, customary law, religious law, and mixed legal systems.
Civil law systems have their origin in the Roman legal tradition. Civil systems vary widely, both in procedure and substantive law, so conducting research on a particular nation’s civil law system should include looking at that nation’s specific system of law, but they do have some trademark characteristics. Nations with civil law systems have comprehensive, frequently updated legal codes. Most importantly, case law is a secondary source in these jurisdictions. France and Germany are two examples of countries with a civil law system.
Common law systems, while they often have statutes, rely more on precedent, judicial decisions that have already been made. Common law systems are adversarial, rather than investigatory, with the judge moderating between two opposing parties. The legal system in the United States is a common law system (with the exception of Louisiana, which has a mix of civil and common law).
Customary law systems are based on patterns of behavior (or customs) that have come to be accepted as legal requirements or rules of conduct within a particular country. The laws of customary legal systems are usually unwritten and are often dispensed by elders, passed down through generations. As such, customary law research depends greatly on the use of secondary sources. Oftentimes, customary law practices can be found in mixed legal system jurisdictions, where they’ve combined with civil or common law.
Religious legal systems are systems where the law emanates from texts or traditions within a given religious tradition. Many Islamic nations have legal systems based in whole or in part on the Quran.
Mixed legal systems refer to legal systems where two or more of the above legal systems work together.