How did Joseph Haydn’s Opus 33 quartets change the string quartet?

Joseph Haydn’s Opus 33 quartets, which he described as being written in a "new manner," fundamentally transformed the string quartet by establishing the principle of thematic development.1

Key innovations introduced in these works include:

• Thematic Development: Haydn shifted the focus to motive manipulation, where the texture of the music is derived from the development of specific motives.1
• Sonata Form: The Opus 33 set features the fully developed sonata form, which became a cornerstone of Classical composition.1
• Structural Changes: Haydn introduced the scherzo as a replacement for the traditional minuet, although he did not maintain this specific substitution in his later quartets.1

These advancements moved the genre away from the earlier, more ornamental styles and provided a structural foundation that influenced later composers, including Wolfgang Amadeus Mozart, who successfully adopted Haydn's new manner in his own works.1