Ternary form is a symmetrical structure in music most often represented by the letters ABA. The A represents a musical idea or ideas, the B represents new, contrasting material, and the final A represents a return to the familiar music heard in the opening of the piece. This structure is important for us to review for two reasons. First, it provides the foundation of the more elaborate sonata-allegro form that develops in the Classical period. Second, in many larger works that feature four movements, such as symphonies and string quartets, the third movement consists of two dance movements, minuet and trio, organized in ternary form (minuet-trio-minuet).

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 de 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 and 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.

Compound Ternary or Trio Form

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.

Figure 1. Diagram of a minuet and trio

The Scherzo and Trio which is identical in structure to other trio forms developed in the late Classical period. Examples include the Scherzo and Trio (second movement) from Beethoven’sSymphony No. 9 and the Scherzo and Trio in Schubert’s String Quintet. Another name for the latter is “composite ternary form.” Trio form movements (especially scherzos) written from the early romantic era sometimes include a short coda (a unique ending to complete the entire movement) and possibly a short introduction. The second movement of Beethoven’s Symphony No. 9 is written in this style which can be diagrammed as [(INTRO) (1A–1A–1B–1B) (2A–2A–2B–2B) (1A–1B) (CODA)]

Polkas are also often in compound ternary form.

Ternary Form within a Ternary Form

In a complex ternary form each section is itself in ternary form in the scheme of [(A–B–A)(C–D–C)(A–B–A)] By convention each part is repeated and only on its first rendition[(A–A–B–B–A)(C–C–D–D–C)(A–B–A)] . An example are the Impromptus (Op. 7) by Jan Voříšek.