6th
Just call them “Their Royal Highnesses”
The current reigning Grand Duke, Henri, has five children: Guillaume, Félix, Louis, Alexandra, and Sébastien. Guillaume is Le Grand-Duc héritie, the next in line for the monarchy.
Why, then, are all five addressed as “His (or Her) Royal Highness”?
To be honest, I don’t completely understand why. Apparently, there’s a difference between “style of address” and “noble title” and each kind of status can be inherited, granted, and sometimes, married into.
But I do know one thing about Luxembourg’s first family: When Grand Duchess Charlotte married Prince Félix, on this day in 1919, Félix brought a little gift for his in-laws. From that day forward, Charlotte’s siblings would be called “Her (or His) Royal Highness” (HRH) rather than the much less impressive “His Grand Ducal Highness” — their previous status.
And ever since, all the children of the reigning Grand Duke or Duchess have been called Royal Highness, regardless of whether they are destined to rule. Charlotte married a younger son of the House of Bourbon-Parma, you see, and the Bourbon-Parmasean are called HRH.
