The ritornellos are guideposts to the tonal structure of the music, confirming the keys to which the music modulates. The first and last statements are in the tonic; at least one (usually the first to be in a new key) is in the dominant; and others may be in closely related keys.

What is the ritornello form and how does it work?

Many Baroque concertos are structured in a form known as ritornello form. In this form, a repeated section of music, the ritornello (literally, "the little thing that returns") alternates with freer episodes. ... The verse and chorus sturcture of modern popular song derives from this form.

What does a ritornello do?

Ritornello, (Italian: “return”) also spelled ritornelle, or ritornel, plural ritornelli, ritornellos, ritornelles, or ritornels, a recurrent musical section that alternates with different episodes of contrasting material. The repetition can be exact or varied to a greater or lesser extent.