development

The development is the central section of sonata form, where the composer takes the themes introduced in the exposition and transforms them — fragmenting, modulating, combining, and recombining them in new and often dramatic ways. It is the most harmonically unstable section, moving through multiple keys. The development section is where a composer's craft and imagination are most clearly on display. Beethoven's developments are famously expansive and intense, sometimes exceeding the exposition in length. Haydn, by contrast, often wrote witty, surprising developments that played with the listener's expectations. The section typically builds tension that resolves with the recapitulation.