Common law the body of customary law based on judicial decisions and embodied in reports of decided cases that has been administered by the courts of england since the middle ages from it has evolved the legal systems found in the united states and most of the commonwealth countries as well. Common law also known as case law is a body of unwritten laws based on legal precedents established by the courts common law draws from institutionalized opinions and interpretations from . Common law n the traditional unwritten law of england based on custom and usage which developed over a thousand years before the founding of the united states the best of the pre saxon compendiums of the common law was reportedly written by a woman queen martia wife of a briton king of a small english kingdom. Common law is the legal system used in great britain and the united states except the state of louisiana according to common law judges must consider the decisions of earlier courts precedents about similar cases when making their own decisions