Lovely warm colours from the lower strings, but where was the rhythmic profile of that surging principal theme?

Once past the liquid principal theme, the third movement bore ahead like a dentist's drill.

The inclusion of the spectacles in this picture turns this process of looking into the principal theme.

The principal theme of the judgment, however, was that the parties were free to make whatever agreement they liked.

In rondo form, a principal theme (sometimes called the refrain) alternates with one or more contrasting themes, generally called episodes, but also occasionally referred to as digressions or couplets.