What musical structure is characterized by a repeated opening passage throughout a movement?

Ritornello form is characterized by the use of a repeated opening passage, which serves as a refrain throughout a movement. In this structure, the main or 'ritornello' theme is typically introduced at the beginning of the movement and alternates with contrasting episodes. This allows for a rich interplay between the main theme and the varied sections, creating a dynamic form that is particularly common in Baroque concertos. The ritornello theme is often presented in its entirety or in fragments during the contrasting episodes, reinforcing the unity of the work while providing opportunities for exploration and contrast. This distinctive characteristic clearly defines ritornello form and differentiates it from other musical structures, which may have different organizational principles.