How did sonata form develop from rounded binary form?

Sonata form evolved from the binary form prominent in the 17th and early 18th centuries, specifically through its expansion and elaboration into rounded binary form.12

In binary form, the structure is defined by the interrelationship of tonalities and themes: the first part moves from the tonic to a new key, and the second part returns to the original key. Rounded binary form refined this by ensuring the second part concludes with a recapitulation of the opening material in the tonic key.13

Sonata form expanded this two-part structure into a more complex three-part organization:12

• Exposition: Corresponds to the first part of binary form, establishing thematic and tonal contrast.1
• Development and Recapitulation: Together correspond to the second part of binary form, where themes are explored, transformed, and ultimately restated in the tonic.12

While sonata form is sometimes called "compound binary form" to highlight these origins, it is distinguished by its emphasis on dramatic conflict and the organic, dialectical treatment of musical themes rather than simple repetition.14