The Royal House reference article from the English Wikipedia on 24-Apr-2004
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Royal House

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The term Royal House refers to the official designation and name of a royal family. Most Royal Families do not have surnames: those that have adopted them rarely use them. They are referred to instead by their titles, often related to an area ruled or once ruled by that family. The name of the Royal House is not a surname, just a way of identifying individuals.

Examples of Royal/Imperial House Names

Cases Where a Royal House has adopted a Surname