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Instructions for the Government of Armies of the United States in the Field (Lieber Code). 24 April 1863.
Section VI : Exchange of prisoners -- Flags of truce -- Flags of protection - Art. 115.
Art. 115. It is customary to designate by certain flags (usually yellow) the hospitals in places which are shelled, so that the besieging enemy may avoid firing on them. The same has been done in battles, when hospitals are situated within the field of the engagement.