sentences:  Since opera libretti have relied on subject matter from the history of literature since the very origin of the genre of opera, a broader usage of the term would cover the entire history of opera, regardless of the underlying libretto structure.According to a seminal publication by Peter Petersen, the term means „a special form of music theater in which the libretto is based on a literary work whose linguistic, semantic and aesthetic structure remains recognizable in the musical-dramatic work as a structural layer.“
  
  The tradition of literaturoper only became established in European opera culture when, with Richard Wagner and the "through-composed dramatic form" he developed, the conventions of verse metrics for the opera libretto had faded.At the same time, the personal union of libretto poet and composer appeared as the new norm of opera production.