How did the modern string quartet emerge around 1750?

The emergence of the modern string quartet around 1750 was driven by a shift in performance practices and the decline of the basso continuo.1

Prior to this period, chamber music was dominated by the trio sonata, which typically featured two violins and a continuo group consisting of a keyboard instrument (such as a harpsichord) and a bass instrument (like a cello or bassoon). As the custom of serenading became popular in Austrian and Italian cities, small groups of musicians began performing in the streets. Because the keyboard instrument used for the continuo was unwieldy for such settings, it was eventually abandoned.1

To maintain the necessary harmonic fullness without the keyboard, a viola was added to the two violins, and the bass line was assigned to the cello. This new configuration of two violins, viola, and cello was subsequently adopted by composers of serious music, with Joseph Haydn playing a pivotal role in establishing the genre as the principal medium for chamber music.12