Based on common law.

That part of the English law not embodied in legislation. It consists of rules of law based on common custom and usage and on judicial decisions. English common law became the basis of law in the US and many other English-speaking countries.
Common law developed after the Norman Conquest 1066 as the law common to the whole of England, rather than local law. As the court system became established (under Henry II), and judges’ decisions became recorded in law reports, the doctrine of precedent developed. This means that, in deciding a particular case, the court must have regard to the principles of law laid down in earlier reported cases on the same, or similar points, although the law may be extended or varied if the facts of the particular case are sufficiently different. Hence, common law (sometimes called “case law” or “judge-made law”) keeps the law in harmony with the needs of the community where no legislation is applicable or where the legislation requires interpretation.