The term and formal principle may have derived from the medieval poetic form, rondeau, which contains repetitions of a couplet separated by longer sections of poetry.
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. 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.
The Baroque predecessor to the rondo was the ritornello. Ritornello form was used in the fast movements of baroque concertos. 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.
A common expansion of rondo form is to combine it with sonata form, to create the sonata rondo form. Here, the second theme acts in a similar way to the second theme group in sonata form by appearing first in a key other than the tonic and later being repeated in the tonic key. Unlike sonata form, thematic development does not need to occur except possibly in the coda.
Examples include the final movement of J.S. Bach‘s E Major Violin Concerto and the last movement of Beethoven‘s Piano Sonata, Op. 13.
Beethoven: Piano Sonata No. 8 in C minor, Op. 13 “Pathetique”
I. Grave. Allegro di molto e con brio II. Adagio cantabile III. Rondo. Allegro