ABSTRACT
During the early years of the Classical period (very approximately 1760–80), the augmented sixth assumed a very specific role: it functioned as a signpost to significant structural boundaries within tonal architecture. This role emerged during the 1730s, and by 1790 had become something of a cliché. The augmented sixth increasingly defined the following chord as dominant, and so, in many respects, its function became more established than for earlier or later generations, but, at the same time, the augmented sixth started to ‘migrate’ to other steps of the scale – particularly the subdominant and flat supertonic. Before looking at the specific structural role in more detail, this chapter first considers some of the remarkable and original ways in which Haydn explored the chord to generate tonal momentum in the development section. Then, after comparing the larger-scale contexts of the chord in two related works by Haydn and Mozart (respectively, Symphony No. 78 and Piano Concerto K. 491) the closing examples illustrate the way in which Mozart’s music of the late 1780s represents the culmination of the eighteenth-century tonal tradition.
