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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. 106.
Art. 106. In exchanging prisoners of war, such numbers of persons of inferior rank may be substituted as an equivalent for one of superior rank as may be agreed upon by cartel, which requires the sanction of the government, or of the commander of the army in the field.