Which musical form was primarily employed in a solo concerto?

The form primarily employed in a solo concerto is ritornello form. This structure is particularly associated with the Baroque concerto, especially in pieces written by composers like Vivaldi and Bach. In a typical ritornello form, the orchestra often presents a recurring theme (the ritornello) that alternates with contrasting solo passages. This creates a dialogue between the soloist and the orchestra, showcasing the soloist's virtuosity while still maintaining a cohesive framework.

Ritornello form allows for the development of the soloist's material alongside the reintroducing orchestral sections, providing both variety and unity throughout the movement. This interaction is essential to the concerto style, making ritornello form a distinctive and effective approach in the solo concerto landscape.