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