Ternary form

Ternary form, sometimes called song form, is a three-part musical form consisting of an opening section (A), a following section (B) and then a repetition of the first section (A). It is usually schematized as A–B–A. Prominent examples include the da capo aria "The trumpet shall sound" from Handel's Messiah, Chopin's Prelude in D♭ Major "Raindrop", (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 aria

Baroque opera arias and a considerable number of baroque sacred music arias was dominated by the Da capo aria which were in the ABA form. A frequent model of the form began with a long A section in a major key, a short B section in a relative minor key mildly developing the thematic material of the A section and then a repetition of the A section. By convention in the third section (the repeat of section A after section B) soloists may add some ornamentation or short improvised variations. In later classical music such changes may have been written into the score.