Customary international humanitarian law is a set of unwritten rules derived from a general, or common, practice which is acknowledged as law. It's the basic standard of conduct in armed conflict accepted by the world community. Customary international humanitarian law is applicable universally – independently of the application of treaty law – and is based on extensive and virtually uniform State practice regarded as law.
In the 1990s, at the request of the international community, the ICRC undertook an extensive study into current State practice in international humanitarian law in order to identify customary law in this area. Before the study was published, no comprehensive written analysis and compilation of customary IHL rules existed. The study has contributed to identifying the common core of international humanitarian law binding on all parties to all armed conflicts. Today, the ICRC’s Customary IHL Database makes these rules and the commentaries thereto easily accessible and searchable.