Annex to the Convention: Regulations respecting the laws and customs of war on land - Section II : Hostilities - Chapter V : Armistices - Regulations: Art. 39.
Art. 39. It rests with the Contracting Parties to settle, in the terms of the armistice, what communications may be held in the theatre of war with the inhabitants and between the inhabitants of one belligerent State and those of the other.