Legal Systems - Guide to International and Foreign Law Research

• While they often have statutes, common law systems 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).

• They 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.

These are the 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.