Haydn, with the assistance of Mozart, established both the string quartet and the symphony as principal modes of musical expression in Western music. Each form was given a new expressive depth and an increased range of technical possibility by these two masters. Ideas from the quartets, especially the establishment of a more or less standard progression of movements, carried over to their work in symphonic form. But there are great internal and emotional differences between the two forms.
One of the principal differences between chamber music and symphonies in the late eighteenth century was that the former was meant for playing and the latter for listening. Works such as the string quartets of Haydn and Mozart were largely meant for the "discerning ears" of groups of skilled amateurs whose private performances might be presented to "small audiences of connoisseurs." But symphonies were written to be enjoyed by the concert-going public. Haydn's symphonies were the popular center of a career that made his fortune and the pieces were very widely played. Mozart's symphonies, however, placed "unusual demands on the performers" and, since they "required a fully professional orchestra," were less frequently played in the eighteenth century than those of Haydn and others. As this makes clear, the complexity of the symphony had only begun to be tapped by these masters but it had already surpassed that possible in the quarter by the middle of Haydn's career.
Haydn was engaged in the attempt to establish the quartet "as an art form capable of conveying profound and original feeling" and he accomplished this with his innovative four-movement sequence (allegro, minuet and trio, minuet, and closing allegro). This replaced the static symmetry of earlier quartets and Haydn further "enriched the content of the quartet" by enlarging "the role of the lower instruments"--eventually following a general rule of equality among the four. ...