|Queen Maxima, Princess Beatrix, King Willem-Alexander|
The Netherlands is in the privileged situation of having a title system that takes care of abdicating monarchs without having thought about the possibility. When Queen Wilhelmina abdicated, she returned to her former title of princess, as did Juliana and Beatrix after her. This easy transition was explained in connection with the title of Queen Maxima. When Willem-Alexander became king, she was automatically addressed as queen. Dutch parliament confirmed that this was correct as an honorary title in keeping with international use, but that under Dutch law she remained Princess Maxima. Only the ruling monarch becomes or is king or queen. Problem solved.
That explains the marked difference to its neighbour Belgium, where there are currently two kings and three queens. Belgium has the system that the ruling king's consort becomes queen. It is only logical, then, that Queen Fabiola retained her title after the death of King Baudouin; and the situation is a well known one anyhow if we remember Elizabeth the Queen Mother (though that title was a creation by decree of Queen Elizabeth II). It was the abdication of King Albert II that made things awkward. He probably wouldn't have given a fig if he had become prince again, or been handed a duchy. But Queen Paola would suddenly have been outranked by Fabiola. Now that would have made life awkward at home. Belgium therefore has currently Queen Fabiola (HM Queen Fabiola), Paola (HM Queen Paola), and Mathilda (HM Queen of the Belgians). There are also two kings; the system applies there, too, with King Albert (HM King Albert) and Philippe (HM King of the Belgians). You might also strive to remember that when place names are necessary in piling up royalty, the ruling king and his consort are never 'of Belgium' but always 'of the Belgians'. Everyone else is 'of Belgium'.
Andorra is in the comfortable position that its princes retire automatically without having to abdicate explicitly. As one prince of the country is the Bishop of Seu d'Urgell in Spain, he ceases to be Prince of Andorra as soon as he retires as bishop. The other prince is the president of France, and they get changed every few years. Happy Andorra doesn't even have to worry about pensions for them. The two remaining hereditary principalities of Monaco and Liechtenstein could deal with the problem easily as the title prince can be used before, during, and after ruling the country.
The quandary the abdication of Pope Benedict XVI caused gives you an indication of how unprecedented it was and how ill prepared monarchies usually are for when it really happens. Having two popes is a bit unusual, but then there were times when we had five popes at the same time gallivanting across Europe. While Pope Emeritus took care of the main title problem, the matter of address hasn't been really solved. Popes usually are addressed as Your Holiness (in writing HH); is Benedict now Your Emeritedness (HE), of maybe Your Former Holiness (HFH)?
The United Kingdom showed one way of dealing with the problem when they dumped a duchy on the head of King Edward VIII upon his abdication. But as the Belgians showed, it doesn't always work. Anyone with clever solutions will probably receive a duchy in any of these countries for solving a very vexing and more and more pressing problem. Even monarchs, after all, are entitled to some retirement.
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