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.

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.

Commonly, the third section will feature more ornamentation than the first section (as is often the case with da capo arias). In these cases the last section is sometimes labeled A’ or A1 to indicate that it is slightly different from the first A section.

In a trio form each section is a dance movement in binary form (two sub-sections which are each repeated) and a contrasting trio movement also in binary form with repeats. An example is the minuet and trio from the Haydn's Surprise Symphony. The minuet consists of one section (1A) which is repeated and a second section (1B) which is also repeated. The trio section follows the same format (2A repeated and 2B repeated). The complete minuet is then played again at the end of the trio represented as: [(1A–1A–1B–1B) (2A–2A–2B–2B) (1A–1A–1B–1B)]. By convention in the second rendition of the minuet, the sections are not repeated with the scheme [(1A–1A–1B–1B) (2A–2A–2B–2B) (1A–1B)]. The trio may also be referred to as a double or as I/II, such as in Bach's polonaise and double (or Polonaise I/II) from his second orchestral suite and his bouree and double (or Bouree I/II) from his second English Suite for harpsichord.

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