Part 1:
Part 2:
Part 3:
Part 4:
At one point, the “Prince Jean has an illegitimate daughter” scenario was not hypothetical, but real.
Grand Duke Jean’s second son, Prince Jean, had his first child, Marie-Gabrielle, born out of wedlock on September 8, 1986. The mother of Marie-Gabrielle was Prince Jean’s French girlfriend Hélène Vestur. They did not marry until their daughter was eight months old.
As explained in Parts 1-3: If the family had simply followed the legal customs of Luxembourg (and the Benelux region in general), Marie-Gabrielle would have been born as Miss Marie-Gabrielle of Luxembourg of Nassau of Bourbon of Parma. She would have been untitled due to being born out of wedlock, but would have carried the legal surname “of Luxembourg of Nassau of Bourbon of Parma”.
But Grand Duke Jean evidently didn’t want Prince Jean’s out-of-wedlock child to have “of Luxembourg of Nassau of Bourbon of Parma” as her legal surname. His Grand-Ducal Decree ensured that did not happen.
On July 28, 1986, when Hélène Vestur was in the final weeks of her pregnancy, Grand Duke Jean signed a Grand-Ducal Decree which read:
“Grand-Ducal Decree of 28 July 1986 concerning the family name of the Princes and Princesses of Luxembourg
In the public and private acts relating to them, the Princes and Princesses, issuing in direct and male-line descendance from the Sovereign and born from a marriage contracted with His consent, carry the title of Princes and Princesses of Luxembourg, following their forenames and preceding the mention of their family name “Nassau”.
The princesses united with the princes of Our Grand-Ducal House by a marriage concluded with the consent of the Sovereign will be referred to in the same manner, following their own forenames, titles and family names.”
The first paragraph of the decree applied to Prince Jean.
A key phrase is “the title of Prince[...] of Luxembourg”. This phrase clarified that “Prince of Luxembourg” should be construed as a title –
not, as custom would have it, a combination of the title “Prince” and the surname “of Luxembourg”.
By declaring that “of Luxembourg” was part of Prince Jean’s title, not part of his surname, the decree ensured that Prince Jean’s illegitimate child could not be called “of Luxembourg”, since illegitimate children do not inherit titles. It seems that is how Grand Duke Jean wanted it.
However, Prince Jean’s untitled child would require a surname. It was not possible to use her mother’s surname of Vestur or to invent a new surname: As of 1986, the civil codes in Luxembourg and France required newborn children to take their father’s legal surname, even if their parents were unmarried.
The decree resolved this issue by stipulating that Prince Jean carried the “family name ‘Nassau’”. Because this “Nassau” was legally a surname, rather than part of a title, Prince Jean’s illegitimate child
was capable of inheriting it. Indeed, seven weeks later she was born as “(Miss) Marie-Gabrielle Nassau.”
(Two decades later, when Prince Louis was expecting an illegitimate child in 2006, Grand Duke Henri
issued another decree to amend the legal family name from “Nassau” to “
of Nassau”.)
I hope this series of posts explains what I meant by Grand Duke Jean separating the surname from the title, the effects the 1986 grand-ducal decree, and why it was necessary to issue the decree so that Marie-Gabrielle would be born as “Marie-Gabrielle Nassau”.

Questions and comments are welcome.