 From the 1950s onward, Hans Keller developed a 'two-dimensional' method of analysis that explicitly considered form and structure from the point of view of "listener expectations".In his work, the sonata-allegro was a well-implied 'background form' against whose various detailed features composers could compose their individual 'foregrounds'; the 'meaningful contradiction' of expected background by unexpected foreground was seen as generating the expressive content.In Keller's writings, this model is applied in detail to Schoenberg's 12-note works as well as the classical tonal repertoire.