 The fact that so-called monothematic expositions usually have additional themes is used by Charles Rosen to illustrate his theory that the Classical sonata form's crucial element is some sort of "dramatization" of the arrival of the dominant.Using a new theme was a very common way to achieve this, but other resources such as changes in texture, salient cadences and so on were also accepted practice.The key of the second subject may be something other than the dominant (for a major-mode sonata movement) or relative major (for a minor-key movement).