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Manual of the Laws of Naval War. Oxford, 9 August 1913.
Section III : On means of injuring the enemy - Art. 28.
Art. 28. In bombardments all useless destruction is forbidden, and especially should all necessary measures be taken by the commander of the attacking force to spare, as far as possible, sacred edifices, buildings used for artistic, scientific, or charitable purposes, historic monuments, hospitals, and places where the sick or wounded are collected. on condition that they are not used at the same time for military purposes.
It is the duty of the inhabitants to indicate such monuments, edifices, or places by visible signs, which shall consist of large stiff rectangular panels divided diagonally into two coloured triangular portions, the upper portion black, the lower portion white.