Encompassing sixty-eight works composed over a span of more than four decades, Haydn's quartet oeuvre contributed to the establishment, solidification, and refinement of late 18th-century chamber-music practices, notably by furnishing superlative models of idiomatic ensemble technique (involving textural diversity and continuous change in relationship among the instruments), style (highlighting the play of formal conventions and musical customs associated with folk music, popular dance, opera, concerto, and other genres), and compositional process (featuring motivic elaboration, harmonic novelty, and narrative intrigue). Conventions Haydn adapted for quartet use include those of sonata form, the minuet-trio complex, variation, rondo, and fugue. In addition, he established norms of his own for the sequence of movements in a quartet and for the design of the opus groups, each of which encompasses a particular constellation of variety and consistency in form, style, and ensemble technique. Examination of the opus groups reveals insights into the circumstances under which they were written, the musical resources on which they drew, their innovations, their points of connection with other opus groups, their manifestations of both change and continuity in outlook and style, and their reflections of Haydn's artistic personality — in particular his penchant for novelty in sonority, theme, metrical play, phraseology, and large-scale structure; his gift for musical irony in drawing connections between seemingly unrelated ideas; and his irrepressible musical wit and humor, typically involving strokes of surprise, thwarted expectation, and the whimsical juxtaposition of incongruous elements.