The two primary systems of American law are the common law system and the civil law system. The common-law system, which developed in England, is the most prevalent system of law in North America. The civil law system in North America is strongest in Louisiana in the United States and in Quebec in Canada; it is also prevalent in Mexico.

The common-law system is based on precedent and the principle of stare decisis. Although the legislative bodies at the federal, state, and local levels enact written statutes, and sometimes collect portions of those statutes into “codes,” there is no formal, comprehensive code of common law. Instead, the common law is stated in court decisions, and it is changed or modified by subsequent cases or statutes.

The civil law system may be traced back to the Roman law from which most European law systems originated. It was brought to the Western Hemisphere by the French, Spanish, and Portuguese. The civil law system as it exists in Europe is the result of Napoleon Bonaparte’s efforts: he provided for the drafting of the Code Civil or Code Napoleon, which restated the earlier principles of Roman law in more modern terms. In the civil law system, the Code Civil is a general statement of legal principles that is looked to in the interpretation of statutes and cases, and civilian courts do not follow the principle of stare decisis. The civil law system also does not have a division between law and equity.