Règle correspondante
Practice Relating to Rule 37. Open Towns and Non-Defended Localities
Section A. Establishment of open towns
The Military Manual (2005) of the Netherlands states:
Parties to a conflict are prohibited from attacking undefended localities. This is a development of the older “open city” doctrine. The authorities of a party to a conflict may designate any inhabited place near the zone in which military operations are being conducted as an undefended locality. This, therefore, is a unilateral declaration.
During the Second World War, cities were several times declared “open”. In June 1940, for example, the French Government declared Paris and Bordeaux, among other places, open cities. In 1945 the Allies declared Rome an open city after landing in Italy. 
Netherlands, Humanitair Oorlogsrecht: Handleiding, Voorschift No. 27-412, Koninklijke Landmacht, Militair Juridische Dienst, 2005, § 0552.