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Ternary form is a three-part A–B–A structure in a piece of music. The first and third parts (A) are musically identical, or very nearly so, while the second part (B) contrasts sharply with it. The B section is often known as a trio.

The first section of a piece in ternary form does not usually change key. However, an example where this is not the case is in Mozart's Piano Concerto No. 21 (K. 467) 2nd movement. In this second movement, the A section is in a different key to the third section. The middle section will generally be in a different key, often the dominant of the first section (a perfect fifth above). 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. Often the trio is in a 3/4 time signature as opposed to the 4/4 of the primary march theme.

Commonly, the third section will feature more ornamentation than the first section (e.g. 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 music era was usually the third movement of symphonies, string quartets, sonatas and similar works.

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."

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