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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. 118.
Art. 118. The besieging belligerent has sometimes requested the besieged to designate the buildings containing collections of works of art, scientific museums, astronomical observatories, or precious libraries, so that their destruction may be avoided as much as possible.