A concubine is a legalized form of mistress in a societal structure that allows a man [who has the money, rank, or privilege to afford it] secondary wives. Usually there is a chief wife who has more status and authority than the others. She is often in charge of the others, the concubines. Often a situation involving royalty and allowing for maximum production of blooded heirs. The Chinese Emperors once had concubines, one of whom rose to be an Empress.