What role does a coda play at the end of a sonata movement?

In a sonata movement, the coda serves as a concluding section that follows the recapitulation to round off the composition. Its primary role is to provide a final resolution to the musical arguments presented throughout the movement, particularly when the conflicts of key and theme established in the exposition are not fully settled by the recapitulation alone.12

Functionally, the coda typically involves the extension or reelaboration of thematic material previously heard. It can range from a brief few measures to a substantial section of significant proportions. Tonally, the coda often introduces subdominant harmony, which acts as a counterbalance to the tonic-dominant relationship emphasized earlier in the movement.1

While the coda is a common feature, it is not considered an essential structural element of sonata-allegro form in the same way as the exposition, development, or recapitulation. Some composers, such as Ludwig van Beethoven, occasionally expanded the coda to such an extent that it functioned almost as a second development section.2