National law is a set of rules or principles applied by courts. Generally, national law is based on the principles and obligations set by a nation or a community of states. However, there are instances where there is no governing body, or when the law is not written down. In these cases, a court or a group of judges must decide on the meaning of a given national law.

Despite this distinction, there are many aspects of national law that are common to many different legal systems. For example, customs are rules of conduct that a nation or state follows when maintaining international relations. These practices become law over time. New customs are created through the efforts of states. For example, the United States and Soviet Union devised new laws regarding space during the Cold War. And the International Criminal Court draws upon many customs when deciding cases.

In some cases, national laws become international laws because they are based on supranational tribunals, such as the European Court of Human Rights. In addition, treaties like the Geneva Convention may require that national laws be compatible with the provisions of the treaty. Further, a national constitution may contain provisions that implement international legal obligations. The distinction between national and international law is difficult to draw, but there is a general consensus that the former is more powerful than the latter.

A-national law is more complicated than domestic law. In addition to being more complex than domestic law, a-national law also requires the skills of a lawyer. For instance, a lawyer should have a thorough understanding of trade usages and unwritten rules.