A royal house or royal dynasty is a designation for a family that has ruled a state for any number of generations. The term may refer to imperial, royal or even noble families, such as dukedoms or earldoms. Modern examples include the British House of Windsor, the Dutch House of Orange-Nassau and the Spanish House of Bourbon. Historically, the most famous royal houses are the German House of Hohenzollern, the French House of Bonaparte and the incredibly long-reigning Austrian House of Habsburg-Lorraine. Though today royal houses serve a figurehead role in most countries, with the exception of the Arabian House of Saud, in the Middle Ages and Renaissance period those monarchies ruled with near absolute power.

Tracking a royal household can be very difficult as different cultures have different qualifications for the continuity of a dynasty and often intermarriages can complicate matters greatly. For instance, the British House of Hanover is actually of German origin, a matter which become especially embarrassing for the royal family when they were fighting Germany in the First World War. Furthermore, new dynasties can often germinate out of old ones, as when the English Houses of Lancaster and York were united under King Henry VII, thereby creating the House of Tudor.

To assist with alternative history writing, here is a list of reigning royal houses (wikipedia):

Similarly, here is a list of extinct royal houses (wikipedia):

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