The common-law legal system forms a major part of the law of many countries, especially those with a history as British territories or colonies. It is notable for the inclusion of extensive non-statutory law reflecting a consensus of centuries of judgements by working jurists.
The common law originally developed under the auspices of the adversarial system in historical England from judicial decisions that were based in tradition, custom, and precedent. The form of reasoning used in common law is known as casuistry or case-based reasoning. Common law may be unwritten or written in statutes or codes. The common law, as applied in civil cases (as distinct from criminal cases), was devised as a means of compensating someone for wrongful acts known as torts, including both intentional torts and torts caused by negligence and as developing the body of law recognizing and regulating contracts. Today common law is generally thought of as applying only to civil disputes; originally it encompassed the criminal law before criminal codes were adopted in most common law jurisdictions in the late 19th century. The type of procedure practiced in common law courts is known as the adversarial system; this is also a development of the common law.
Before the institutional stability imposed on England by William the Conqueror in 1066, English citizens were governed by unwritten local customs that varied from community to community and were enforced in often arbitrary fashion. For example, courts generally consisted of informal public assemblies that weighed conflicting claims in a case and, if unable to reach a decision, might require an accused to test guilt or innocence by carrying a red-hot iron or snatching a stone from a caldron of boiling water or some other "test" of veracity (trial by ordeal). If the defendant's wound healed within a prescribed period, he was set free as innocent; if not, execution usually followed.
In 1154, Henry II became the first Plantagenet king. Among many achievements, Henry institutionalized common law by creating a unified system of law "common" to the country through incorporating and elevating local custom to the national, ending local control and peculiarities, eliminating arbitrary remedies, and reinstating a jury system of citizens sworn on oath to investigate reliably criminal accusations and civil claims. The jury reached its verdict through evaluating common local knowledge, not necessarily through the presentation of evidence, a distinguishing factor from today's civil and criminal court systems.