From The Art and Popular Culture Encyclopedia
The rondeau (French; plural form rondeaux) was a Medieval and early Renaissance musical form, based on the contemporary popular poetic rondeau form. It is distinct from the 18th century rondo, though the terms are likely related. With the virelai and the ballade, it is one of the three formes fixes of French music and poetry in the 14th and 15th centuries.
The rondeau form calls for a rigid pattern of repetition of verse and refrain, following the evolving rhyme-scheme of the poetic form, ranging from eight lines to as many as 21. The most commonly used form is ABaAabAB, where capital letters refer to repetition of the (two-part) refrain text and music while lowercase letters refer to repetition of music alone with a new text. The following short song by Machaut, "Doulz viaire gracieus", illustrates the pattern in its simplest, 8-line form, where each musical part corresponds to a single line of lyrics.