It is usually in the dominant, subdominant, parallel, or relative key of the musical work's main key, but also in any variation or combination of them; the subdominant of the relative major is common in Classical-era minor-key sonatas.
If the middle part is written in a major key, it is often labeled Maggiore.
This theme is usually eight to thirty-two bars in length, and may be constructed as a musical sentence, period, or small ternary.
Each variation is a recurrence of the theme with melodic, harmonic, rhythmic and ornamental changes.
It may bring back the original theme with few or no changes, in order to create symmetry.