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Royal mistress
http://en.wikipedia.org/wiki/Royal_mistress
A royal mistress is the historical position of a mistress to a monarch or an heir apparent. Some mistresses have had considerable power. These powerful mistresses have been commonly referred to as the "power behind the throne". The prevalence of the institution can be attributed to the fact that royal marriages were until recent times conducted solely on the basis of political and dynastic considerations, leaving little space for the monarch's personal preferences in the choice of a mate.
In European history the children of mistresses were not normally included in the line of succession, except perhaps when secret marriages were alleged. Hence the Monmouth Rebellion when James Scott, 1st Duke of Monmouth claimed the thrones of England and Scotland on the grounds that his mother had been the wife, rather than a mistress of Charles II.
Lists of Royal mistresses
http://en.wikipedia.org/wiki/Category:Royal_mistresses
http://en.wikipedia.org/wiki/Royal_mistress#Royal_mistresses_in_Eng...
http://en.wikipedia.org/wiki/Ma%C3%AEtresse-en-titre
http://en.wikipedia.org/wiki/List_of_French_royal_mistresses