 If it is extended, it is, in general, slower than the main section and frequently focuses on the dominant key.It may or may not contain material that is later stated in the exposition.The introduction increases the weight of the movement (such as the famous dissonant introduction to Mozart's "Dissonance" Quartet, KV 465), and also permits the composer to begin the exposition with a theme that would be too light to start on its own, as in Haydn's Symphony No.