<Tr> <Th> Major key </Th> <Td> </Td> <Td> V </Td> <Td> </Td> <Td> VI, IV or parallel minor </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Th> Minor key </Th> <Td> </Td> <Td> III or V </Td> <Td> </Td> <Td> VI or IV </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <P> In rondo form, a principal theme (sometimes called the "refrain") alternates with one or more contrasting themes, generally called "episodes", but also occasionally referred to as "digressions" or "couplets". Possible patterns in the Classical period include: ABA, ABACA, or ABACABA . These are sometimes designated "first rondo", "second rondo", and "third rondo", respectively . The first rondo is distinguished from the three - part song form principally by the fact that at least one of the themes is a song form in itself, but the difference in melodic and rhythmic content of the themes in the rondo form is usually greater than in the song form, and the accompanimental figuration in the parts of the rondo (unlike the song form) is usually contrasted . The number of themes can vary from piece to piece, and the recurring element is sometimes embellished and / or shortened in order to provide for variation . </P> <P> A Baroque predecessor to the rondo was the ritornello . Ritornello form was used in the fast movements of baroque concertos, and in many baroque vocal and choral works . The entire orchestra (in Italian, tutti) plays the main ritornello theme, while soloists play the intervening episodes . While Rondo form is similar to ritornello form, it is different in that ritornello brings back the subject or main theme in fragments and in different keys, but the rondo brings back its theme complete and in the same key . Cedric Thorpe Davie is one author, however, who considers the ritornello form the ancestor, not of the rondo form, but of the classical concerto form (which also occurs, as a form, in many a classical - era aria .) </P>

A b a c a b a is an example of which form from the classic period