Starting mostly in the 20th century, a number of monarchies were deposed. However, to this day we can still see members of ousted royal families being called by the titles they held when they once reigned.
It is important to note that deposed royals obviously don’t have their titles recognised in the countries they used to reign, and when other people, or even themselves, style them by these titles it is a matter of courtesy and a way to show respect for the role they used to perform.
As a general rule, when a King is deposed, he keeps the title of King to the day he dies, but his heir, who would someday succeed him as monarch, continues to be styled by a title usually held by the heir to the throne, e.g., when Peter II, the former King of Serbia, died, his son continued to use the title of Crown Prince.
It is also important to observe that the present royal courts of reigning monarchies accord deposed royals the same rank they would have if they were still on the throne. For instance, when Her Serene Highness Princess Caroline of Monaco married her now estranged husband, His Royal Highness The Prince of Hanover, the Prince’s Palace of Monaco started to style her as Her Royal Highness The Princess of Hanover. The Hanover title has no legal standing, but it follows the tradition of having women adopting their husband’s titles if the husband’s rank is higher than hers, and that would be the case if Hanover had remained a monarchy.
Usually, deposed royal families and the governments from their respective countries don’t easily reach an agreement as to the possession of former royal properties, but there are some cases where these families are able to retain ownership of their palaces. That was the case in both Romania and Serbia when both families had their palaces restored to them after the fall of the communist regimes.
In Germany, since 1919, hereditary titles are only allowed as part of one’s surname. The previously mentioned His Royal Highness The Prince of Hanover’s name according to German Law is Ernst August Prinz von Hannover.
The former Greek Royal Family occupies a particularly interesting position. King George I of the Hellenes, the first Greek monarch from the Schleswig-Holstein-Sonderburg-Glücksburg dynasty, was the son of King Christian IX of Denmark, and as such, he and his descendants all held the title of Prince of Denmark and were even in the line of succession to the Danish Throne. To this day, members of the deposed Greek Royal Family are considered to hold the title of Prince or Princess of Denmark through their descent from King Christian IX. However, all Greek dynasts lost their rights of succession to the Danish throne when the law changed in 1953.
Adding to that, Queen Anne-Marie, who is married to former King Constantine, is the youngest sister of Queen Margrethe II of Denmark, who considers Anne-Marie and her descendants as members of the extended Danish Royal Family. As such, the Greek royals are typically at important critical royal events in Denmark and celebrate Christmas there every few years.