How did Johann Sebastian Bach change the role of the keyboard in sonatas?

Johann Sebastian Bach played a pivotal role in elevating the keyboard from its traditional, subordinate function in the Baroque continuo to a primary, independent voice in the sonata.1

In the earlier Baroque tradition, the keyboard was typically used for "filling-in" harmonies, often improvised over a figured bass. Bach moved away from this limited capacity by composing works that integrated the keyboard as an essential, equal partner. Specifically, he produced:1

• Obbligato Harpsichord Sonatas: Bach wrote sonatas for violin, flute, or viol with obbligato harpsichord parts, meaning the keyboard accompaniment was fully written out and obligatory rather than improvised.1
• Solo Keyboard Sonatas: He contributed to the development of sonatas written specifically for keyboard instrument alone, building upon the work of predecessors like Johann Kuhnau.1

By treating the keyboard as a fully realized melodic and harmonic participant, Bach helped shift the focus toward the exhaustively detailed music notation and specific instrumental scoring that would eventually define the Classical era.1