Origins and History
The origins of sonata-allegro form grew from older Baroque forms of the 17th century and early 18th century. The shift in styles at the century's turn had a large part in the culmination of the sonata-allegro form as well. In the Baroque period of the 17th century, a musical movement or song focused on a single emotional idea. Each movement was a mediation on one feeling. Works in this style created interest and drama through the contrasting emotions of the different movements. For example, a work with several movements would alternate between slow, sad movements and then fast, happy movements.
The stylistic shift from the older Baroque period to the Classical period of the 18th century focused on changing and varied emotions within a piece along with contrasting music within one movement. From this desire for contrast within a movement came the practice of writing two themes in the Exposition in contrasting styles and contrasting tonalities.
As the century developed, certain habits for sonata-allegro form became cemented through repeated use. Composers did not write music to fit the form. Instead the form developed through the composers’ music.
How originality brings a standardized form.
One stylistic mark of the Classical period remains the interest in defying the audience's expectation. Composers wrote music full of irony by establishing expectation in the audience of the music sounding certain way only to give a different result. One manner in which composers set up expectations was through the established norms of form. From these musical clichés, sonata-allegro form developed.
Even as the form became established throughout Europe, the desire to defy expectation remained. Therefore, composers did not write music to fit into the cookie-cutter rules of sonata-allegro form. The rules were used instead a guidelines. In fact, as the form became more and more established, composers, beginning with Beethoven at the early 19th century and continuing on, ventured further and further away from the most fundamental aspects of the form, including the tonal basis, discernable themes and thematic organization.