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The Act on Membership of the Royal House included transitional provisions to exempt royals who were already born from the new rules, in some cases.
Article 3.
1. They who, at the time this law enters into force, are an an adult member of the Royal House and capable of succeeding the King under the terms of the Constitution retain their membership of the Royal House. They retain their membership for as long as they are capable of succeeding the King under the terms of the Constitution.
2. The spouse of one who retains their membership of the Royal House in accordance with Paragraph 1 also retains their membership.
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At the time the Act entered into force in 2002, Margriet and Constantijn were adult members of the Royal House and in line to succeed the King under the Constitution. Therefore, by virtue of Article 3(1), Margriet and Constantijn “retain their membership for as long as they are capable of succeeding the King under the terms of the Constitution.” Under Article 3(2), so do their spouses.
The Constitution fixes the third degree of consangunity from the King as the limit of the line of succession to the Kingship. If and when Catharina-Amalia is Queen (King), Margriet will no longer be “capable of succeeding the King under the terms of the Constitution”, since Margriet is only a fourth-degree relative of Catharina-Amalia. Therefore, Margriet will be excluded from the Royal House.
Alexia and Ariane were not yet born in 2002. The exception created by Article 3 therefore will not apply to them, and the normal rule in Article 1 will be valid: They will be excluded from the Royal House if they are no longer first- or second-degree relatives of the monarch. That will happen if and when a child of Catharina-Amalia takes the throne.
There is also a transitory provision for titles and surnames:
Article 10
They who bear titles and surnames under the terms of the Royal Decrees of 26 October 1937 (State Bulletin of 1937, number 5) and 2 January 1967 (State Bulletin of 1967, number 1) shall retain them.
The Royal Decree of 26 October 1937 laid down the surname and (according to the then government’s interpretation) the titles of Juliana and Bernhard’s future children (Beatrix, Irene, Margriet, and Christina).
The Royal Decree of 2 January 1967 laid down the titles and surnames of Margriet and Pieter’s future children (Maurits, Bernhard, Pieter-Christiaan and Floris).
Article 10 permits them to retain for life their titles and surnames granted by those royal decrees, irrespective of what happens to their membership of the Royal House. That is why Irene and Christina remain Princess of the Netherlands despite having left the Royal House before the Act was passed.
But Article 10 does not cover Constantijn's, Alexia or Ariane's titles. For them the normal rule in Article 8(3) that “The title ‘Prince (Princess) of the Netherlands’ expires upon the loss of membership of the Royal House” will apply.