Main theme
The most important theme of a movement.
What it means
The main theme is the central musical idea of a movement — the idea that determines the character, returns most often, and forms the starting point of the working-out. In the sonata movement it is the first theme of the exposition, in the home key; as a section of the form this part is called the Hauptsatz (first theme group).
Musical use
What makes a theme the main theme is less its position than its significance for the movement. It is as a rule pregnant enough to be recognized and at the same time rich enough in motifs to be worked out. Characteristic rhythms, striking intervals, and a clear harmonic basis contribute to this.
In the sonata movement it is opposed by a second theme in another key. The older study of form liked to describe this relationship as an opposition of two characters; in practice it is more varied, and not seldom the second theme is derived from the main theme. In the development the main theme is often broken up, and its motifs appear in changing contexts; in the recapitulation it returns in the home key.
In other forms the main theme has a corresponding role: in the rondo as the recurring refrain, in the variation as the starting point of the changes, in the fugue as the subject that is taken up by all the voices.
The main theme is to be distinguished from the motif, the smaller building block from which themes are formed, and from the identifying motif, which carries an extra-musical meaning.
In fugue, rondo, and variations the main theme stands at a central place and returns in different form. In sonata movements there is with the second theme a counterpart that can be contrasting or related. For listeners the main theme is the anchor by which they follow the course of the movement.
Performance practice
Impress the main theme and its motifs on yourself so that you recognize them in every working-out. Shape it with a clear profile but without exaggeration — it should carry, not show off. And make its return audible, especially when it happens in altered shape.