In the classical era, there were fewer and fewer oratorios and cantinas composed, but the biggest difference was the subject matter of the opera. Baroque operas were based on classical mythology, but in the classical era they shifted to more realistic contemporary subject matter reflecting the Enlightenment ideas present in the classical era. Some major differences between the operas of these two eras, however, were subject matter and form. During the Baroque era, opera seria, or serious opera, was very common. Opera seria was characterized by its rigid form and its appeal to authority or people in power. Later Baroque opera emphasized strophic variation on the da capo aria which was established as a vocal form. Alongside these forms was the bipartite aria, consisting only of A and B variations. Unlike late Baroque opera and its rigid alternation of recitative and aria, Middle Baroque opera retained great flexibility formal. The change from this rigid and serious form of opera occurred in the years 1762 to 1783, when the introduction of a simpler, more humane form of opera was introduced. This change may be somewhat contributed to Christoph