The most prominent development in musical form in the Classical Period was the creation of Sonata-Allegro Form.
Think of sonata form as the musical equivalent to the story arc of a movie or a play.
The movie opens. We’re introduced to the setting (introduction), followed by its main characters (theme 1 & theme 2). Tensions rises, and some sort of crisis occurs and reaches its climax (development). Then the crisis is resolved (recapitulation). Maybe there’s a twist at the end (coda).
The mains “scenes” within sonata-allegro form are 1) exposition, 2) development, and 3) recapitulation. Each does just as their names suggest. The exposition exposes us to the main themes of the piece. The first theme has a stronger, heroic character with a fast tempo. The second theme is slower with a sweeping lyrical character. Think in terms of the Romeo & Juliet, if the first theme is the families fighting, the second theme is the love theme between Romeo and Juliet. Musically, each of the themes is composed in different keys, the tonic (home key), and its dominant (creator of tension; the beginnings of our “crisis”).
As the tension of the piece rises, we enter the development, where the composer takes the themes presented to us in the exposition and places them in new keys, rhythms, and contexts to create tension and instability. Once we reach the climax of the development, we enter into the recapitulation, where the tension created in the development in resolved. The recapitulation is exactly the same music as the exposition, except for one element: the second theme stays in our home key. Since we are resolving the “story,” there is no need to present the theme in the dominant key that introduces tension into the storyline. We want to stay home and come to a strong resting point, our final cadence. To drive the point home, composer may also add a coda. The coda repeats the main themes, or may develop them further (Beethoven loves to do this). Always with a strong declaration, and repetition of ideas that tells us, the listener, we have arrived at our destination.
Listen to the first movement of Mozart’s Symphony no. 40 in G minor, and follow along with the timeline below:
Here are some other sonata form timelines worth checking out:
One final note, the first movement of any symphony or string quartet of the Classical Period is always in sonata form.