Through the power of precedent, international incidents involving the use of force help to clarify the meaning and interpretation of jus ad bellum, the corpus of rules arising from international custom and the United Nations Charter that govern the use of force. UN Charter Article 2(4) forbids states from using force in their international relations. Exceptions to this prohibition are acts taken in self-defence under UN Charter Article 51 or under the auspices of a UN Security Council authorization to use force under Article 42. States can also consent that another state use force in its territory, for example to combat rebel or terrorist actors. In certain cases, state practice gives rise to new interpretations of existing rules or novel exceptions emerge.