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. In recent times, two other musicologists, James Hepokoski and Warren Darcy, have presented, without reference to Keller, their analysis, which they term Sonata Theory, of the sonata-allegro form and the sonata cycle in terms of genre expectations, and categorized both the sonata-allegro movement and the sonata cycle by the compositional choices made to respect or depart from conventions. Their study focuses on the normative period of sonata practice, notable ones being the works of Haydn, Mozart, Beethoven, Schubert, and their close contemporaries, projecting this practice forward to development of the sonata-allegro form into the 19th and 20th centuries.