Some common factors which were pointed to include: the shift of focus from vocal music to instrumental music; changes in performance practice, including the loss of the continuo.Crucial to most interpretations of the sonata form is the idea of a tonal center; and, as the Grove Concise Dictionary of Music puts it: "The main form of the group embodying the sonata principle, the most important principle of musical structure from the Classical period to the 20th century: that material first stated in a complementary key be restated in the home key". (The sonata idea has been thoroughly explored by William Newman in his monumental three-volume work Sonata in the Classic Era (A History of the Sonata Idea), begun in the 1950s and published in what has become the standard edition of all three volumes in 1972. 20th-century theory
Heinrich Schenker argued that there was an Urlinie or basic tonal melody, and a basic bass figuration. He held that when these two were present, there was basic structure, and that the sonata represented this basic structure in a whole work with a process known as interruption.As a practical matter, Schenker applied his ideas to the editing of the piano sonatas of Beethoven, using original manuscripts and his own theories to "correct" the available sources. The basic procedure was the use of tonal theory to infer meaning from available sources as part of the critical process, even to the extent of completing works left unfinished by their composers.
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.