There was also in the early Classical period the possibility of using four movements, with a dance movement inserted before the slow movement, as in Haydns Piano sonatas No. 6 and No. 8. Mozarts sonatas were also primarily in three movements. Of the works that Haydn labelled piano sonata, divertimento, or partita in Hob XIV, seven are in two movements, thirty-five are in three, and three are in four; and there are several in three or four movements whose authenticity is listed as "doubtful." Composers such as Boccherini would publish sonatas for piano and obbligato instrument with an optional third movement—–in Boccherinis case, 28 cello sonatas. But increasingly instrumental works were laid out in four, not three movements, a practice seen first in string quartets and symphonies, and reaching the sonata proper in the early sonatas of Beethoven. However, two- and three-movement sonatas continued to be written throughout the Classical period: Beethovens opus 102 pair has a two-movement C major sonata and a three-movement D major sonata. Nevertheless, works with fewer or more than four movements were increasingly felt to be exceptions; they were labelled as having movements "omitted," or as having "extra" movements. Thus, the four-movement layout was by this point standard for the string quartet, and overwhelmingly the most common for the symphony. The usual order of the four movements was:

An allegro, which by this point was in what is called sonata form, complete with exposition, development, and recapitulation. A slow movement, an Andante, an Adagio or a Largo.
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.