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British royal family
In fact, the British monarchs rarely use their surnames because they don't really need them. They are sufficiently known and without such formalities.
By 1917, the members of the royal family had done quite well without the last name, using only the first name and the name of their dynasty, while King George V did not lodge a change in tradition.
The reason was that King George belonged to the dynasty of Saxe-Coburg and Gotha. It was of German descent, which led to unpleasant associations at a time when Germany was opposed to England in the First World War.
The king decided to change the unpopularly sounding name to something more satisfying, choosing Windsor by the name of one of the castles owned by the royal family. However, it does not only change the name of the dynasty, but also makes Windsor officially the surname of the entire British royal family.
In 1947, Princess Elizabeth
Married Philip Mountbatten, Prince of Greece and Denmark, who served in the British Royal Navy. The result is that the royal family's current name is Mountbatten-Windsor, although it is not compulsory for its members to use it.
In certain cases this is necessary (e.g. school education), but even then they have a choice. For example, Prince William and Prince Harry served in the Royal army under the name of Wales because their father was a Prince of Wales. The Queen's other grandchildren (the children of Princess Ann) took the name from her father, Phillips. And another granddaughter of the Queen, the daughter of Prince Edward, uses the name Winsor, without the added Mountbatten.
It is not difficult to lose hopeless in these multiple names and titles. Therefore, and it is no surprise that today, as well as 100 years ago, members of the royal family deal without a last name, whenever they can.