Characteristics
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 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. 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.
Read more about this topic: Ternary Form