Ratio decidendi

Ratio decidendi (US: /ˌreɪʃioʊ ˌdɪsaɪˈdɛndi, -daɪ/; Latin plural rationes decidendi) is a Latin phrase meaning "the reason" or "the rationale for the decision". The ratio decidendi is "the point in a case that determines the judgement" or "the principle that the case establishes".

In a court judgment, the ratio decidendi is the legal rule derived from, and consistent with, those parts of its reasoning on which the outcome of the case depends. It refers to the legal, moral, political, and social principles used by the court to compose the rationale of a particular judgment. In contrast to obiter dicta, the ratio decidendi is usually binding on lower courts through the doctrine of stare decisis. Certain courts can overrule decisions of a court of coordinate jurisdiction, but they generally try to follow earlier rationes out of interests of judicial comity.

The process of determining the ratio decidendi is an analysis of what the court actually decided, based on the legal points about which the parties in the case actually fought. All other statements about the law in the text of the judgment — all pronouncements that do not form a part of the court's ruling on the issues decided in that particular case (whether they are correct statements of law or not) — are obiter dicta, and are not rules on which that particular case stands.