The______ is an essential characteristic of the sonata form.
http://en.wikipedia.org/wiki/Sonata#The_sonata_idea_or_principle
http://en.wikipedia.org/wiki/Sonata_form
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There are about a hundred terms that can fit here. I am not certain any of them are "essential", but some would argue that. If I had to pick one term, it would be "musical theme". I doubt if that is what your teacher wants.
The essential characteristic of the sonata form is the "exposition." To determine this, we can break down the sonata form into its basic components and analyze their roles.
1. Exposition: This is the first section of the sonata form. It introduces the principal themes of the musical piece, typically consisting of two contrasting themes – the first theme (often in the tonic key) and the second theme (usually in a contrasting key). The exposition establishes the musical material that will be developed throughout the composition.
2. Development: Following the exposition, the development section takes place. Here, the composer explores and manipulates the themes introduced in the exposition. This section often involves modulation, fragmentation, variation, and other techniques to create musical tension and excitement.
3. Recapitulation: After the development section, the music returns to the original themes presented in the exposition. However, in the recapitulation, both the first and second themes are usually restated in the tonic key. This brings a sense of resolution and unity to the composition.
4. Coda: Lastly, the coda serves as a concluding section that provides closure to the sonata form. It often contains material derived from the previous sections and may include new thematic material as well.
Therefore, the exposition is the essential characteristic of the sonata form as it sets up the primary themes and establishes the foundation for the development and recapitulation sections.