Most nations today follow one of two major legal traditions: common law or civil law. The common law tradition emerged in England during the Middle Ages and was applied within British colonies across continents. The civil law tradition developed in continental Europe at the same time and was applied in the colonies of European imperial powers such as Spain and Portugal. Civil law was also adopted in the nineteenth and twentieth centuries by countries formerly possessing distinctive legal traditions, such as Russia and Japan, that sought to reform their legal systems in order to gain economic and political power comparable to that of Western European nation-states.
Common law is generally uncodified. This means that there is no comprehensive compilation of legal rules and statutes. While common law does rely on some scattered statutes, which are legislative decisions, it is largely based on precedent, meaning the judicial decisions that have already been made in similar cases. These precedents are maintained over time through the records of the courts as well as historically documented in collections of case law known as yearbooks and reports. The precedents to be applied in the decision of each new case are determined by the presiding judge. As a result, judges have an enormous role in shaping American and British law. Common law functions as an adversarial system, a contest between two opposing parties before a judge who moderates.