A king might have numerous mistresses but have a single `` favourite mistress '' or `` official mistress '' ( in French , `` maîtresse en titre '' ) , as with Louis XV and Madame de Pompadour .
A king might hold numerous mistresses but have a single `` favourite mistress '' or `` official mistress '' ( in French , `` maîtresse en titre '' ) , as with Louis XV and Mme de Pompadour .