Body of ancient law of Rome codified under the Emperor Justinian in the fifth century c.e. The civil law systems of modern Europe and elsewhere, especially the Code Napoleon (1804), use Roman law as their foundation.

Roman law is known for its logical, systematic character, as opposed to the comparatively haphazard nature of common law, which takes previous court decisions as sources for its governing principles. Roman and civil-law systems look instead to the text of rationally constructed legal codes in deciding cases.

Although U.S. jurisprudence is based on the English common-law tradition, Roman law played a key role in its development. Thomas Jefferson urged its study, and to varying degrees, it found its way into American legal education. Its role, however, was long unrecognized. The legal systems of several U.S. territories and states, including Arkansas, Texas, Florida, and Louisiana, were based on civil law. Roman law provided a model of rational legal structure, a source of precise terminology, and a comparative basis for judging common law rules and practices.

Roman law influenced Supreme Court justices, especially from the late eighteenth to the early twentieth century. In particular, it interested Justices Joseph Story and Oliver Wendell Holmes and was often cited in Court decisions. However, the influence of Roman law and its civil-law offspring never threatened the dominance of common law on the Court. As a common-law court, the Court adheres to precedent, following the stare decisis rule. Under civil-law procedures and principles, the Court’s decisions would be justified primarily by their basis in the Constitution or statue law, not on past decisions.

Civil law

Common law

Constitutional law