Sonata-allegro form is a structural pattern used by composers first in the 18th century as a means to organize their music. Similar to a basic essay format of introduction with a thesis, supporting body paragraphs and conclusion that restates the thesis, sonata-allegro form organizes music through an initial statement, development of themes and a recapitulation of the original material. While the origins are much older, sonata-allegro form grew to prominence as a defining characteristic of the Classical style as used by Haydn and Mozart and then further developed by Beethoven.
In an era called the Age of Reason, the aesthetic values of 18th century music emphasize logic and clarity with traits of symmetrical phrases, declarative melodies and simple accompaniment. Organization in music as well grew to prominence because of the growing interest in logic and clarity which allowed for the development of sonata-allegro form.