From The Art and Popular Culture Encyclopedia
Rondo, and its French equivalent rondeau, is a word that has been used in music in a number of ways, most often in reference to a musical form, but also to a character-type that is distinct from the form. Although now called rondo form, the form started off in the Baroque period as the ritornello, from the Italian word ritornare meaning "to return" - indicating the return to the original theme or motif ("A"). The typical Baroque ritornello pattern is ABACABA. Although there are a few differences, some people use the two terms, rondo and ritornello, interchangeably.
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 ABACADA. The number of themes can vary from piece to piece, and the recurring element is sometimes embellished or shortened in order to provide for variation.
The form began to be commonly used during Classical period, though it can be found in earlier works. In the Classical and Romantic periods it was often used for the last movement of a sonata, symphony, concerto or piece of chamber music.
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. But ritornello does differ slightly from other rondos in that the theme is often different when it recurs but is always distinguishable as the same theme.
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.
Rondo as a character-type (as distinct from the form) refers to music that is fast and vivacious—normally allegro. Many classical rondos feature music of a popular or folk character. Music that has been designated as "rondo" normally subscribes to both the form and character. On the other hand, there are many examples of slow and reflective works that are rondo in form but not in character. They include Mozart's Rondo in A minor k511.
A little-known vocal genre of the late eighteenth-century referred to at that time as the "rondò" is cast in two parts, slow-fast.