This is one of the most classic and widely cited definitions of International Law.
It was given by the influential German jurist Lassa Francis Lawrence Oppenheim in his seminal work, "International Law: A Treatise.
" This definition highlights two key sources of international law:
Customary rules: Practices that states follow out of a sense of legal obligation.
Conventional rules: Rules that are agreed upon in treaties and conventions.
Oppenheim's definition emphasizes that these rules govern the relationships ("intercourse") between states.