Ternary form

Ternary form, sometimes called song form, is a three-part musical form where the first section (A) is repeated after the second section (B) ends. It is usually schematized as A–B–A. Examples include the da capo aria "The trumpet shall sound" from Handel's Messiah, Chopin's Prelude in D-Flat Major (Op. 28) and the opening chorus of Bach's St John Passion.

Simple ternary form

In ternary form each section is self-contained both thematically as well as tonally (that is, each section contains distinct and complete themes, and ends with an authentic cadence. The B section is generally in a contrasting but closely related key, usually a perfect fifth above or the parallel minor of the home key of the A section (V or i); however, in many works of the Classical period, the B section stays in tonic but has contrasting thematic material. It usually also has a contrasting character; for example section A might be stiff and formal while the contrasting B section would be melodious and flowing. Da capo arias are usually in simple ternary form.