<P> The ritornello was also crucial in the development of the Italian instrumental concerto during the Baroque period . Giuseppe Torelli wrote many violin concertos in which the fast movements used a recurring ritornello in between two extended solo passages of entirely new material . This form was standardized by Antonio Vivaldi, who wrote hundreds of concertos using a modification of Torelli's scheme . Vivaldi's ritornello form established a set of conventions followed by later composers in the eighteenth century: </P> <Ul> <Li> Ritornellos for the full orchestra alternate with episodes for the soloist or soloists . </Li> <Li> 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 . </Li> <Li> Later statements of the ritornello are usually partial, comprising only one or some of the units, sometimes varied . </Li> <Li> 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 . </Li> </Ul> <Li> Ritornellos for the full orchestra alternate with episodes for the soloist or soloists . </Li> <Li> 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 . </Li>

Who plays the contrasting sections in a ritornello