A libretto (Italian: 'little book') is the text of an opera, oratorio, cantata or musical — the words that are set to music. The relationship between librettist and composer is one of the great creative partnerships in art history: Lorenzo Da Ponte wrote the texts of Le nozze di Figaro, Don Giovanni and Così fan tutte for Mozart in a single productive period in the 1780s; Hugo von Hofmannsthal wrote five libretti for Richard Strauss, including Der Rosenkavalier and Elektra; W.S. Gilbert and Arthur Sullivan produced their operettas as a perfect collaboration between wit and music. The librettist's task is to supply words that are rhythmically flexible, dramatically viable, singable and intelligible in performance — an almost impossibly constrained creative act. Verdi famously manipulated his librettists, while Wagner wrote his own.