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Rules concerning the Control of Wireless Telegraphy in Time of War and Air Warfare. Drafted by a Commission of Jurists at the Hague, December 1922 - February 1923.
Part II: Rules of Air Warfare#Chapter IV: Hostilities#Bombardment. - Art. 25.
Art. 25. In bombardments by aircraft, all necessary steps should be taken by the commander to spare, as far as possible, buildings dedicated to public worship, art, science, and charitable purposes, historic monuments, hospital ships, hospitals and other places where the sick and wounded are gathered, provided that such buildings, objectives and places are not being used at the same time for military purposes. Such monuments, objects and places must be indicated, during the day, by signs visible from the aircraft. Using such signs to indicate buildings, objects or places other than those hereinbefore specified shall be considered a perfidious act. The signs of which the above mentioned use is to be made, shall be, in the case of buildings protected under the Geneva Convention, the red cross on a white ground and, in the case of the other protected buildings, a large rectangular panel divided diagonally into two triangles, the one white and the other black.
A belligerent who desired to ensure by night the protection of hospitals and other above mentioned privileged buildings, must take the necessary steps to make the aforesaid special signs sufficiently visible.