All legal systems deal with the same basic issues, but each country categorises and identifies its legal subjects in different ways. A common distinction is that between "public law" (a term related closely to the state, and including constitutional, administrative and criminal law), and "private law" (which covers contract, tort and property).^[8] In civil law systems, contract and tort fall under a general law of obligations, while trusts law is dealt with under statutory regimes or international conventions. International, constitutional and administrative law, criminal law, contract, tort, property law and trusts are regarded as the "traditional core subjects",^[9] although there are many further disciplines which may be of greater practical importance.

In law, custom can be described as the established patterns of behavior that can be objectively verified within a particular social setting. A claim can be carried out in defense of "what has always been done and accepted by law." Generally, customary law exists where:

1. a certain legal practice is observed and
2. the relevant actors consider it to be law

The modern codification of civil law developed out of the customs, or coutumes of the Middle Ages, expressions of law that developed in particular communities and slowly collected and later written down by local jurists. Such customs acquired the force of law when they became the undisputed rule by which certain entitlements (rights) or obligations were regulated between members of a community.^[1] The Custom of Paris - the customary law that developed within the city of Paris - is an example of custom law.

In international law, customary law refers to the Law of Nations or the legal norms that have developed through the customary exchanges between states over time, whether based on diplomacy or aggression. Essentially, legal obligations are believed to arise between states to carry out their affairs consistently with past accepted conduct. These customs can also change based on the acceptance or rejection by states of particular acts. Some principles of customary law have achieved the force of peremptory norms, which cannot be violated or altered except by a norm of comparable strength. These norms are said to gain their strength from universal acceptance, such as the prohibitions against genocide and slavery. Customary international law can be distinguished from treaty law, which consists of explicit agreements between nations to assume obligations. However, many treaties are attempts to codify pre-existing customary law