Binary form, in music, the structural pattern of many songs and instrumental pieces, primarily from the 17th to the 19th century, characterized by two complementary, related sections of more or less equal duration that may be represented schematically as ab. In 18th-century compositions, including dance-inspired movements by J.S. Bach and keyboard sonatas by Domenico Scarlatti, the two sections are separated by double bars with repeat signs, so that a proper performance actually yields an aabb structure.

The first section of a binary composition in a major key typically modulates to the dominant, thus displacing the centre of harmonic gravity to the fifth degree above the tonic: compositions in minor keys similarly modulate to the relative major (i.e., the major key centred on the third degree above the tonic). The second section begins in the new key and, after thriving for a period on the harmonic thus generated, returns to the home key. Binary structures, though not necessarily monothematic, tend to rely on closely linked melodic-rhythmic materials.

In the “rounded” binary form, as exemplified by many of Scarlatti’s sonatas, the second section returns rather quickly to both the original key and the melodic-rhythmic features of substantial portions of the first section. By the same token, binary organization of this sort begins to approximate the ternary outline of a number of later 18th- and 19th-century pieces.

Ternary form

Ternary form, in music, a form consisting of three sections, the third section normally either a literal or a varied repeat of the first. The symmetrical construction of this scheme (aba) provides one of the familiar shapes in Western music; ternary form can be found in music from the Middle Ages (as in the common arrangement antiphon-verse-antiphon in Gregorian chant) to the present day.

Although any kind of aba pattern may be correctly defined as ternary, the term most precisely denotes the form exemplified by the minuet and trio of the Baroque suite and the Classical symphony and sonata, as well as the da capo aria of the Baroque cantata and oratorio and 18th-century opera. In the Classical minuet, the minuet section and the trio section must each comprise at least a period or a double period and must end on an authentic cadence; that is, each section is relatively complete within itself. The trio section follows the minuet and is usually in a different key. Then the minuet is repeated; this repetition may be indicated by the term da capo, “from the head”), or it may be written out in full, especially if it is varied in some way. Ludwig van Beethoven and his successors usually replaced the symphonic minuet with a scherzo, a movement similar in form to the minuet but much faster in tempo.

The standard aba is often described as a simple ternary form, as distinct from a compound ternary form, which may be abacaba or abacdaba with the c or the cd in a different key; this pattern approximates rondo form (in which a particular melody or section is periodically restated).

Sonata form has sometimes been considered an expanded category of ternary form, with its three sections of exposition, development, and recapitulation, but this characterization is misleading. Sonata form, the most highly developed of the Classical forms, in fact evolved historically from binary form into a more-complex structure that belongs in a distinct category of its own.