Rules of fair play, originally developed by the courts of equity to control the decisions of inferior courts and then gradually extended (particularly in the 20th century) to apply equally to the decisions of administrative and domestic tribunals and of any authority exercising an administrative power that affects a person's status, rights, or liabilities. Any decision reached in contravention of natural justice is void as ultra vires. There are two principal rules. The first is the rule against bias (i.e. against departure from the standard of even‐handed justice required of those who occupy judicial office)—nemo judex in causa sua (or in propria causa) (no man may be a judge in his own cause). This means that any decision, however fair it may seem, is invalid if made by a person with any financial or other interest in the outcome or any known bias that might have affected his impartiality. The second rule is known as audi alteram partem (hear the other side). It states that a decision cannot stand unless the person directly affected by it was given a fair opportunity both to state his case and to know and answer the other side's case.
The rules of natural justice provide a minimum standard of procedural fairness; the exact requirements will vary depending on the context.