<P> Tracing Haydn's work over the six decades in which it was produced (roughly from 1749 to 1802), one finds a gradual but steady increase in complexity and musical sophistication, which developed as Haydn learned from his own experience and that of his colleagues . Several important landmarks have been observed in the evolution of Haydn's musical style . </P> <P> In the late 1760s and early 1770s, Haydn entered a stylistic period known as "Sturm und Drang" ("storm and stress"). This term is taken from a literary movement of about the same time, though it appears that the musical development actually preceded the literary one by a few years . The musical language of this period is similar to what went before, but it is deployed in work that is more intensely expressive, especially in the works in minor keys . James Webster describes the works of this period as "longer, more passionate, and more daring ." Some of the most famous compositions of this time are the "Trauer" (Mourning) Symphony No. 44, "Farewell" Symphony No. 45, the piano sonata in C minor (Hob . XVI / 20, L. 33), and the six string quartets of Op. 20 (the "Sun" quartets), all from c. 1771--72 . It was also around this time that Haydn became interested in writing fugues in the Baroque style, and three of the Op. 20 quartets end with a fugue . </P> <P> Following the climax of the "Sturm und Drang", Haydn returned to a lighter, more overtly entertaining style . There are no quartets from this period, and the symphonies take on new features: the scoring often includes trumpets and timpani . These changes are often related to a major shift in Haydn's professional duties, which moved him away from "pure" music and toward the production of comic operas . Several of the operas were Haydn's own work (see List of operas by Joseph Haydn); these are seldom performed today . Haydn sometimes recycled his opera music in symphonic works, which helped him continue his career as a symphonist during this hectic decade . </P> <P> In 1779, an important change in Haydn's contract permitted him to publish his compositions without prior authorization from his employer . This may have encouraged Haydn to rekindle his career as a composer of "pure" music . The change made itself felt most dramatically in 1781, when Haydn published the six string quartets of Opus 33, announcing (in a letter to potential purchasers) that they were written in "a new and completely special way". Charles Rosen has argued that this assertion on Haydn's part was not just sales talk, but meant quite seriously; and he points out a number of important advances in Haydn's compositional technique that appear in these quartets, advances that mark the advent of the Classical style in full flower . These include a fluid form of phrasing, in which each motif emerges from the previous one without interruption, the practice of letting accompanying material evolve into melodic material, and a kind of "Classical counterpoint" in which each instrumental part maintains its own integrity . These traits continue in the many quartets that Haydn wrote after Opus 33 . </P>

Haydn's most important contributions to vocal music were his