Nobiliary titles are personal,it is established at the resolution of the court of Strasbourg of 2000, but also, is included in the agreement of Vienna of 1815. The nobiliary titles are not of the states, because
The royal houses are before than the states. The Royal houses are Personal and historical entities,( with own and particular rules between royal houses outside the states).
The states is a territorial entity, with a population and a political form,. A state can modify its political form, monarchy,or republic, but the Royal house can not be suppressed by a state because it is not competent. A state can grant legal effect to titles nobiliary or not.
The members of the Royal house of the hellenes, have the right among royal houses, among monarchists... to use their title nobilliary can even make use in other states that have recognized valid to titles, the International Olympic Committee has recognized efficacy to titles nobiliary (Constantine is identified as HM Constantine). In the Republics, nobiliary titles have no legal effect. The noble title of prince of Greece and Denmark, is not a state title, is a personal title, "of Denmark" , is competent the Royal House of Denmark, is a historical title, which serves to identify the male offsprings of King Christian.