Why is sonata form not simply a type of ternary form?

While sonata form is sometimes mistakenly categorized as a type of ternary form due to its three-part structure—exposition, development, and recapitulation—this characterization is misleading.12

Sonata form is distinct from ternary form for several key reasons:

• Historical Origins: Unlike ternary form, which is defined by a simple A-B-A pattern of lyrical or character contrast, sonata form evolved from binary form. It functions as a complex, organic structure rather than a symmetrical repetition of sections.12
• Tonal Relationships: In ternary form, sections are often relatively independent. In contrast, sonata form relies on the interrelationship of tonalities. The exposition moves from an original key to a new key, the development explores various keys, and the recapitulation returns to the original key, echoing the structural motion of binary form.1
• Dialectical Development: Sonata form is considered a higher, more complex type of organization. In the development section, musical themes are not merely contrasted; they are treated dialectically—combined, broken up, and transformed—creating a sense of conflict and resolution that is not required in the simpler A-B-A structure of ternary form.1

Because of these complexities, sonata form is more accurately described as a unique, highly developed structure rather than a simple three-part form.12