Annex to the Convention: Regulations respecting the laws and customs of war on land - Section II : Hostilities - Chapter I : Means of injuring the enemy, sieges, and bombardments - Regulations: Art. 27.
Art. 27. In sieges and bombardments all necessary steps must be taken to spare, as far as possible, buildings dedicated to religion, art, science, or charitable purposes, historic monuments, hospitals, and places where the sick and wounded are collected, provided they are not being used at the time for military purposes.
It is the duty of the besieged to indicate the presence of such buildings or places by distinctive and visible signs, which shall be notified to the enemy beforehand.