The document discusses the structure and history of the da capo aria, a three-part operatic aria that became formalized in the mid-17th century. The da capo aria follows an ABA format, with a first section giving way to a contrasting middle section, and then a repetition of the first section to end. This last repeated section allows the performer to demonstrate their full vocal skills and techniques through embellishments and improvisations. While the aria became more formulaic over time, improvisation was restored in many productions starting in the mid-20th century.