Ternary form, sometimes called song form, is a three-part musical form, usually schematized as A–B–A. The first and third parts (A) are musically identical, or very nearly so, while the second part (B) in some way provides a contrast with them. The B section is often called the trio, especially in minuets and scherzi. Examples include Schumann's "Folk Song", Album for the Young (Op. 68, No. 9) and Chopin's Prelude in D-Flat Major (Op. 28).
In ternary form, unlike the rounded binary form, each section will usually be 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 the dominant key of the first section's tonic (a perfect fifth above) or the relative or parallel minor or major, less often the subdominant key (a perfect fifth below). It usually also has a contrasting character; in a march, for example, the highly rhythmic and strident character of the march itself is usually contrasted with a more lyrical and flowing trio in the subdominant, and often in a 3/4 time signature as opposed to the 4/4 of the primary march theme.
A distinction is sometimes made[by whom?] between "compound ternary form" – in which each large part of the form is itself divided in a way to suggest ternary or binary form (giving, for example, an overall scheme of A–B–A–C–D–C–A–B–A) – and "simple ternary form", in which each large part of the form has no particular structure itself. Da capo arias are usually in simple ternary form; minuets (or scherzos) and trios are normally compound [(1A–1A–1B–1B) (2A–2A-2B-2B) (1A-1B)]. Another name for the latter is "composite ternary form".
The repetition of any section results in expanded 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.
As well as in marches, ternary form is often found in baroque opera arias (the da capo aria) and in many dance forms, such as polkas. It is also the form used in the minuet (or scherzo) and trio, which, in the Classical era, was usually the third movement of symphonies, string quartets, sonatas and similar works.
See also 
- "Binary and ternary form" in the Harvard Dictionary of Music, 2nd ed. rev. and enlarged (1969). Willi Apel, ed. Cambridge, Mass.: The Belknap Press of Harvard University Press
- See "Trio (2)" in the Harvard Dictionary of Music, 2nd ed. rev. and enlarged (1969). Willi Apel, ed. Cambridge, Mass.: The Belknap Press of Harvard University Press
- White, John D. (1976). The Analysis of Music, pp. 53–54. ISBN 0-13-033233-X.
- Benward & Saker (2003). Music: In Theory and Practice, Vol. I, p. 315. Seventh Edition. ISBN 978-0-07-294262-0.
- Small Ternary Form www.artofcomposing.com