How did Joseph Haydn help establish the string quartet as a major musical form?

Joseph Haydn played a pivotal role in establishing the string quartet as a major musical form by evolving it from its early, lighter roots into a sophisticated genre defined by thematic development and structural balance.12

His contributions include:

• Standardizing Form: Haydn regularized the four-movement structure, which became the standard for the genre.12
• Equalizing Texture: In his early works, the first violin dominated while other instruments provided accompaniment. By his Opus 33 quartets (1781), he achieved a mature style characterized by the equal participation of all four instruments—two violins, viola, and cello.1
• Implementing Sonata Principles: He infused the quartet with the sonata principle, utilizing motive manipulation to create a "musical drama" within the first movement. This technique of thematic development became a cornerstone of the Classical style.312
• Expanding Expressiveness: Through his extensive output of 68 quartets, Haydn explored increasing harmonic complexity, chromaticism, and emotional depth, setting a high standard of craftsmanship that influenced contemporaries like Wolfgang Amadeus Mozart and Ludwig van Beethoven.312