How does rondo form alternate a main theme with contrasting episodes?

In music, the rondo is an instrumental form defined by the alternation of a recurring main theme—often called a refrain or ritornello—with contrasting sections known as episodes.123

The structure functions through the following principles:

• Refrain and Episode: The main theme (represented as a) is stated initially and returns periodically throughout the piece. Between these statements, the composer introduces contrasting musical material, referred to as episodes (represented as b, c, etc.).13
• Common Schemes: While the number of episodes can vary, most rondos follow established patterns such as the five-part (abaca) or the seven-part (abacaba) scheme.13
• Structural Variations: Composers often adapt the form to suit the work. For example, in the sonata-rondo, the episode typically replaced by c may instead be a development passage based on the main theme.13
• Historical Context: The rondo evolved from the French Baroque rondeau, which utilized a chainlike structure of refrains and couplets. It became a staple of the Classical period, frequently serving as the final movement in symphonies, sonatas, and concerti.13