How does the tonal plan of binary form resemble the tonal plan of sonata form?

The tonal plan of sonata form is directly rooted in the binary form prominent during the 17th and 18th centuries. Both structures rely on a fundamental harmonic journey that moves away from and eventually returns to the original tonic key.12

In binary form, the first section establishes the home key before modulating to a new key—typically the dominant or relative major—where it concludes. The second section begins in that new key, explores various harmonic paths, and ultimately returns to the original tonic.134

Sonata form expands upon this two-part tonal logic:
* Exposition: Corresponds to the first part of binary form, moving from the tonic to a new key.12
* Development and Recapitulation: Together, these sections correspond to the second part of binary form. The development section passes through multiple keys, while the recapitulation provides the necessary return to the original tonic key, mirroring the harmonic resolution found in binary structures.12

Because of this shared foundation, sonata form is sometimes referred to as "compound binary form," highlighting its origin in the earlier, simpler tonal plan.1