Comity is not law because countries do not regard it as something they are obligated to respect. There is no formal law-making machinery. General principles of law. Teachings of publicists. Treaties and Conventions - the most important sources of international law.
Treaties are agreements between one or more nations.
Because one country fears that if it does not respect its promises, other countries will not respect their promises. Legal Status of Eastern Greenland 3. If a state persistently refuses to observe a customary practice from the practice's beginning, it doesn't have to. The Asylum Case 4. The legal principles common to nation states.