Short answers, pulled from the story.
Haydn's String Quartets Op. 20 are a set of six string quartets composed in 1772. They are considered a milestone in the history of composition because Haydn developed techniques in them that defined the medium for the next two centuries, including new approaches to counterpoint, equality of voices among the four instruments, and structural innovations in sonata form.
Haydn composed the Op. 20 quartets in 1772 while serving as Kapellmeister at the Eszterháza palace of Prince Nikolaus Esterhazy, located southeast of Vienna in Hungary. He was forty years old at the time.
The Op. 20 quartets became known as the "Sun" quartets because an image of a rising sun appeared on the cover of an early edition of the score. The first known publication was issued in 1774 by Louis-Balthazar de La Chevardière in Paris.
Beethoven studied the scores of Haydn's Op. 20 quartets before composing his own first set of string quartets, Op. 18, copying them out and scoring the first quartet for string orchestra. In a letter written in 1812, three years after Haydn's death, Beethoven wrote that Handel, Haydn, and Mozart were entitled to their laurel wreaths, and that he himself was not yet entitled to his.
Johannes Brahms owned the autograph manuscript of the Op. 20 quartets, studied them carefully, and annotated them. He bequeathed the manuscript to the Society of Friends of Music in Vienna, where it is preserved today.
Musicologist Roger Hickman has argued that while Haydn may be considered the father of the Classical string quartet, he is not the creator of the string quartet genre itself, since other composers were already writing works in the modern quartet format alongside and before him. Hickman noted that accepting Haydn as sole inventor distorts the achievements of other composers writing in the genre at the same time.