Ritornellos for the full orchestra alternate with episodes for the soloist or soloists.
The opening ritornello is composed of several small units, typically two to four measures in length, some of which may be repeated or varied. These segments can be separated from each other or combined in new ways without losing their identity as the ritornello.
- Later statements of the ritornello are usually partial, comprising only one or some of the units, sometimes varied.
- 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 (usually the first to be in a new key) is in the dominant: and others may be in closely related keys.
- The solo episodes are virtuosic, idiomatic playing, sometimes repeating or varying elements from the ritornello, but often presenting scales, arpeggiations, or other figuration. Many episodes modulate to a new key, which is then confirmed by the following ritornello. Sometimes the soloist interrupts or plays some part of the closing ritornello.
- These points are illustrated by the two fast movements in Vivaldi's Concerto for Violin and Orchestra in A Minor, op.3 no.6. Each of the segments have a strong character that makes it easy to remember. Each is separate harmonic unit, enabling Vivaldi to separate and recombine the segments later on. In both movements, later statements of the ritornello are only partial, and some vary motives from the original ritornello, as do some of the solo episodes. One ritornello in the finale even changes keys, which in most movements only happens during episodes. New figurations are introduced in the episodes, providing even more variety within a clearly understood structure; one passage exploits the open strings of the violin for impressive leaps. Typical of Vivaldi, the alternations between tutti and solo does not stop when the music returns to the tonic near the end of the movement in the first movement, episodes appear between successive nuts of the ritornello, and in the finale, the orchestra and soloist alternate in presenting segments of the final ritornello.
- The result in each case is a movement unique in form, yet the overall strategy is clearly the same. Far from following a textbook plan, Vivaldi's ritornello structures show almost infinite variety in form and content.
- Vivaldi was the first concerto composer to make the slow movements as important as the fast ones. His slow movement is typically a long-breathed expressive, canticle melody, like an adagio operatic aria or arioso, to whose already rich figuration the performer was expected to add embellishments. Some slow movements are through-composed, and others use a simplified ritornello or two-part form. The slow movements in Op.3 No.6 is unusual in that the bass instruments and continuo are silent, and the soloist is accompanied only by the upper strings playing sustained tones.