1) International Comity (Courtesy) :
The Comity of Nations is not a source of International Law, but many former rule of international committee is nowadays rule of international law. Since mutual relations of nations are often based on the principles of comity, International comity has helped the development of international law. When a State behaves in a particular way with other States and the other states also reciprocate and behaves in the same way, there is a comity between the two and dispute is resolved which help in development of international law.
2) International State papers other than treaties :
In modern period, almost all the civilized States have diplomatic relationship with each other. They send letters to each other in respect of matters of mutual interests. These letters are sometimes published. A study of these letters reveals that certain principles are repeatedly followed by States in their mutual intercourse. Sometimes these State papers help in the solution of a conflict or controversy.
3) State Guidance for their officers :
The acts or declarations by statesmen or opinions of legal Advisors to State Governments or official jurists given in relation to particular matters referred to them by their own Governments, or other State papers of International import all furnish evidence of usages grow into International Customs in course of time as they are adapted by other States.
4) Place of Reason in the modern system :
Reason has occupied a special position in all ages . In modern period also it occupied an important place. It has performed a special role in the Development of International Law.
5) The United Nations :
United Nations has made an indirect impact on the development of International Law.
Main Sources of International Law