Why is Joseph Haydn associated with the development of the Classical style?

Joseph Haydn is considered a pivotal figure in the development of the Classical style due to his profound influence on the evolution of musical form and orchestration during the 18th century.12

He was instrumental in establishing the standard forms and styles for the symphony and the string quartet. Throughout his career, Haydn demonstrated a remarkable growth in craftsmanship, moving from early works influenced by his predecessors to complex, mature compositions that defined the era. His contributions include:123

• Formal Innovation: He helped define the harmonic basis of Classical form, the dramatic use of key relationships, and the structural principles of sonata form.3
• Orchestration: He expanded the expressive capabilities of wind instruments and utilized the orchestra to create greater color contrast and dynamic variety.34
• Stylistic Synthesis: Haydn acted as an eclectic architect, amalgamating various styles of his time—including folk music, contrapuntal techniques, and homophony—into unique, expressive shapes that influenced later composers like Mozart and Beethoven.3