In the Sonata form, the second section is traditionally a Development section in which the composer develops the various themes or ideas he introduced in the Exposition section in an ever-changing key area.
If the entire exposition is based on only two main keys, that is, major tonic and its major dominant, or minor tonic and its relative major (for example, C minor and E flat major in the first movement of Beethoven's Fifth Symphony), the Development section compensates for it by bringing forth a series of modulations in which the developed musical ideas appear.
