Sonata form is commonly used in the first movements of symphonies. In Ludwig van Beethoven's Symphony No. 3 in E-flat major a first section (A) in the main key of E-flat major in bars 1 to 83 followed by the traditional second section (B) in the dominant key (B-flat major) in bars 83 to 155. However, section A includes another exposition with its two sections: bars 1 to 45 in the main key (we will call this section Aa) and bars 45 to 83 in the dominant key (we will call it section Ab).

This image illustrates the form of the exposition:

• Sections Ab and B are in the dominant key in the exposition and in the main key in the recapitulation.
• The importance of section Ab is reinforced by the fact that its themes will be used extensively in the development and the coda while the theme of section B will not be developed.

Follows the first movement of the Symphony in E flat major known as Eroica:

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