“Haydn is often referred to as "Papa" Haydn, and there is some justification for this.  Although he did not invent either the symphony or the string quartet, he was a central figure in their development.  His symphonies (especially the "Paris" and "London" sets) show a complete mastery of form and substance, setting the stage for the works of Mozart and Beethoven. Haydn's "paternity" is just as clear in the string quartets. Here Haydn helped transform the genre from little more than a string divertimento (with the emphasis on the top voice) to a type of chamber music in which all parts play an equal role. These ideas directly influenced Mozart, who responded with six quartets dedicated to Haydn (1782-1785). Haydn's style is detectable in the early music of Beethoven as well.”

 – EssentialsofMusic.com