Droit du seigneur
In Mozart's "The Marriage of Figaro", which premiered in 1786 with a libretto by Lorenzo Da Ponte, the comic plot revolves around the successful efforts of the young bride and groom, Figaro and Susanna, to block the efforts of the unfaithful Count Almaviva to seduce Susanna. To achieve his aim, the frustrated Count threatens to reinstitute "droit du seigneur". It was based on a play by Pierre Beaumarchais.
Libretto
By the 20th century some librettists became recognised as part of famous collaborations, as with Gilbert and Sullivan or Rodgers and Hammerstein. Today the composer (past or present) of the musical score to an opera or operetta is usually given top billing for the completed work, and the writer of the lyrics relegated to second place or a mere footnote, a notable exception being Gertrude Stein, who received top billing for "Four Saints in Three Acts". Another exception was Alberto Franchetti's 1906 opera "La figlia di Iorio" which was a close rendering of a highly successful play by its librettist, Gabriele D'Annunzio, a celebrated Italian poet, novelist and dramatist of the day. In some cases, the operatic adaptation has become more famous than the literary text on which it was based, as with Claude Debussy's "Pelléas et Mélisande" after a play by Maurice Maeterlinck.