Scholarship and musicology
Sonata idea or principle
Research into the practice and meaning of sonata form, style, and structure has been the motivation for important theoretical works by Heinrich Schenker, Arnold Schoenberg, and Charles Rosen among others; and the pedagogy of music continued to rest on an understanding and application of the rules of sonata form as almost two centuries of development in practice and theory had codified it. The development of the classical style and its norms of composition formed the basis for much of the music theory of the 19th and 20th centuries. As an overarching formal principle, sonata was accorded the same central status as Baroque fugue; generations of composers, instrumentalists, and audiences were guided by this understanding of sonata as an enduring and dominant principle in Western music. The sonata idea begins before the term had taken on its present importance, along with the evolution of the Classical periods changing norms. The reasons for these changes, and how they relate to the evolving sense of a new formal order in music, is a matter to which research is devoted.
The term came to apply both to the structure of individual movements (see Sonata form and History of sonata form) and to the layout of the movements in a multi-movement work. In the transition to the Classical period there were several names given to multimovement works, including divertimento, serenade, and partita, many of which are now regarded effectively as sonatas. The usage of sonata as the standard term for such works began somewhere in the 1770s. Haydn labels his first piano sonata as such in 1771, after which the term divertimento is used sparingly in his output. The term sonata was increasingly applied to either a work for keyboard alone (see piano sonata), or for keyboard and one other instrument, often the violin or cello. It was less and less frequently applied to works with more than two instrumentalists; for example, piano trios were not often labelled sonata for piano, violin, and cello. Initially the most common layout of movements was:

Allegro, which at the time was understood to mean not only a tempo, but also some degree of "working out", or development, of the theme. A middle movement, most frequently a slow movement: an Andante, an Adagio or a Largo; or less frequently a Minuet or Theme and Variations form. A closing movement was generally an Allegro or a Presto, often labeled Finale. The form was often a Rondo or Minuet.However, two-movement layouts also occur, a practice Haydn uses as late as the 1790s.