Excluded from aulic poetry (which from 14th century up to the early 19th century only used hendecasyllables and their relative hemistichs) they re-appeared in poetry for music in the frottola at the end of the 15th century and the canzonetta in the early 17th century. Senarios and decasyllables, respectively of dactylic and anapestic scansion, were soon added to iambic-trochaic lines as developed by Chiabrera. This addition was probably brought about to fulfill requirements of musical nature.
Subsequently the history of the short-medium length verses is associated with that of the opera libretto, and is characterized by many phases leading up to the standardization established by Metastasio. For its part, after the middle of the century, the composer normally adopts rhythmic patterns extended two bars, with coincidence of two principal stress of the verse with the first beat of the bars. This trend would subsequently lead to regular «square periods». The new system ensures an automatic correspondence between musical and metrical accents. Rare prosodic infringements attested by the literature are caused almost exclusively by irregularities of versification.
But when in 19th century the melody became radically isorhythmical (with instructions for constant rhythmic incipit) both the poetic and musical metric systems, having already reached high degrees of compatibility, showed strong oppositional arrangements or new interactions.
Abstract