Binary Form

Binary form divides a piece into two sections, each typically repeated: ||: A :||: B :||. The defining characteristic is the harmonic relationship between the two sections. In the most common type — called simple binary — the A section ends on the dominant (or in the relative major if the piece is in minor), and the B section works its way back to the tonic. Neither section returns to the opening theme in the tonic at its conclusion. This key-to-dominant-and-back motion is the structural argument of simple binary form, common in Baroque dance movements.

Rounded binary adds a significant refinement: near the end of the B section, the opening theme returns in the tonic. This return — called the "reprise" — gives rounded binary a hint of the ABA logic found in ternary form, but it differs because the reprise is embedded within the B section rather than forming an independent third section. The return of the A theme in the tonic provides a strong sense of resolution, making rounded binary feel more closed than simple binary.

Balanced binary is a third type in which the end of the A section and the end of the B section share the same closing music, now transposed to the tonic. The parallel endings create a sense of large-scale harmonic balance — the same cadential formula that closed section A in the dominant is recapitulated in section B in the tonic. Baroque dance forms (allemandes, courantes, gigues) and short instrumental pieces by composers like Handel and D. Scarlatti are the primary repertoire for binary analysis. Binary thinking also underlies sonata form, which can be understood as an expanded rounded binary.