A legal custom is the established pattern 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".

Customary law (also, consuetudinary or unofficial law) exists where:

1. a certain legal practice is observed and
2. the relevant actors consider it to be an opinion of law or necessity (opinio juris).

Most customary laws deal with standards of the community that have been long-established in a given locale. However, the term can also apply to areas of international law where certain standards have been nearly universal in their acceptance as correct bases of action – for example, laws against piracy or slavery (see hostis humani generis). In many, though not all instances, customary laws will have supportive court rulings and case law that have evolved over time to give additional weight to their rule as law and also to demonstrate the trajectory of evolution (if any) in the interpretation of such law by relevant courts.